Mediums: General Comments

Editor’s note: Mediums and solvents are covered in Virgil’s book, pages 132 -142.

As I stated in my book, glazes made by adding much medium to oil paints will be very fragile and problematic. From a structural strength standpoint, glazes hold up better when they’re made with oil paints whose pigments are transparent by their nature, because those don’t need to be mixed with medium in order to be transparent.

As in masonry, structural strength is greater when the solid particles (pigments in paint; stones in masonry) fit closely together with a minimum of binding material (oil and medium in paint; mortar in masonry) between them. Too high a percentage of mortar and too few stones in masonry results in a weak structure, and by this same principle, too much medium or oil and too few particles of solid matter in a paint film makes for a physically less durable paint film, which will be more vulnerable to damage and more likely to develop defects as it ages.
If a transparentizing medium is desired, one that contains inert solid matter, such as calcium carbonate or barium sulfate would be structurally stronger than oil alone or a fluid medium, because oil shrinks as it cures and ages, and without solid matter to bolster it, the film can pull apart as it shrinks, creating cracks.

Another note: “What is too often overlooked in questions about and discussions of mediums is what paints people are using. Some oil paints are too stiff for good control as they come from the tube, and that’s the main reason why painters think they need to add medium. I suggest investigating other brands of oil paint that are not so stiff before looking into mediums to soften oil paints that are the consistency of window putty, as I mentioned in my book.A big reason for the overly stiff consistency of some brands of oil paints is the stabilizer added in their manufacture. There are brands that use very little stabilizer, and one brand that uses none. With these paints, there is not such a great need to add mediums to soften them in order to make them controllable, or if any medium is needed, less of it is required than with stiffer paints. Other reasons for adding mediums, such as to speed drying or to increase transparency are valid, but from a standpoint of structural integrity of the resulting paint films, it’s best to add no more than is absolutely necessary to achieve the desired optical effects.”

If the reason for wanting to add medium is to create transparent passages (glazing,) that objective is best served by choosing paints whose pigments are transparent by their nature, and applying them thinly. Once again, this is discussed in my book, along with lists of paints distinguished from one another according to whether they are transparent, opaque, or somewhere in between. Creating transparent passages from transparent pigments is a better way than adding medium to opaque paints to transparentize them.

Virgil’s preference:

Any medium should be used with restraint. 10-15% of any mixture should be the upper limit.  Weak paint films result from too little pigment and too much binder. The usual results include cracking as the binder (medium and/or oil) shrinks. It might not be noticeable for years, but it can be expected to happen. Once the paint cracks, other problems begin to develop. Using too much medium is a bad practice unless you don’t care how long your paintings will last. And if you sell them, you should care, because the people who buy them will care.

My medium now is just water-washed, sun-bleached linseed oil, and not very much of that.

Glazes can be made without any added medium if the paints chosen for that purpose are transparent by their nature. Apply them thinly enough to make them as transparent as desired.

A Facebook reader asks: ” I’m curious about the long term effects of using safflower, walnut and linseed oils in a painting. I have tubed colours from different manufacturers, some of which are linseed oil, some safflower and some walnut oil. Any ideas about how using these might behave in the long run when mixed together?”

Virgil replies:  

The different binding oils are compatible with one another in one-layer painting. In multi-layer painting, it’s best to stick with linseed oil paints for all but the final layer, for several reasons. One is drying time, and film strength is another, for both of which linseed oil is superior to the others. In the top layer is where walnut, safflower or poppy oil can be used with more confidence than in the earlier stages; however, poppy oil dries very slowly and is more crack-prone on aging, thus my personal preference is not to use it for anything. Others will probably have different opinions of it, of course.

As for the concerns over linseed oil yellowing, it need not really be as big an issue as many people seem to feel it is, because it’s only a temporary stage that reverses if the painting is exposed to light. But for those who are worried about it, walnut oil is a good choice for the colors where yellowing would be most noticeable, the blues and white. Safflower oil paints are probably all right for those colors as well.

Linseed Oil

Virgil’s assessment: 

Linseed oil is a good medium for adjusting the consistency of oil paints.

Compared to walnut oil,  “linseed oil dries better and faster, and makes stronger, more durable paint films. For multi-layer techniques, linseed oil paints are best for all but the final layer. Walnut and safflower oils yellow less, so are often used as binders for blues, where yellowing would be more noticeable than it would in other colors.”

The yellowing of linseed oil paints is a temporary condition that reverses with exposure to light.

Alkali-refined linseed oil is good. So is cold-pressed linseed oil, and perhaps if it’s water-washed it’s even better. Procedure for water-washing linseed oil is in my book, or you can buy some already done that way by Rolf Hareem of RGH oil paints.

Linseed oil is usually available in large quantities at hardware stores and paint stores less expensively than in art supply stores or health food stores. In health food stores it’s called Flax Oil or Flaxseed Oil, for use as a nutritional supplement since it’s high in omega 3 fatty acids. But if it has vitamin E or other preservatives in it, it’s not suitable for art purposes because it won’t dry, so read the label before you buy.

Note that hardware store linseed oil is best used only for brush cleaning, as it’s less pure than artist-grade linseed oil, and therefore more likely to darken as it ages. So don’t use hardware store linseed oil for a painting medium or ingredient in a medium.

Stand Oil

Virgil’s assessment:

Stand oil is a polymerized linseed oil that is generally used as an ingredient in oil painting mediums, most often in mixtures with turpentine or odorless mineral spirits. It imparts a “long” brushing quality to the paints to which it is added, making them brush out like enamel. Long paints work well on smooth surfaces such as panels and fine-weave, triple-primed canvases, in conjunction with soft-hair brushes like sables, sabeline, ox hair, etc.

Among its few drawbacks are that it tends to add gloss to the painting, which may be objectionable if unevenly distributed because there will be more gloss in the strokes that contain more of it than in the strokes that have less of it, and the thicker varieties of it require thinning with a solvent, the vapors of which are harmful to breathe. With adequate ventilation, the potential for harm can be reduced. The gloss is not as great a concern in smaller paintings if they are properly lit when displayed but can be a problem with larger size paintings unless the lighting comes from a high angle from above. A coat of varnish will usually even out the surface gloss.

Stand oil mediums work well with umbers, which dry matte unless a glossy medium is added to compensate for that characteristic of umber pigments, and of siennas to a lesser degree. (Note that when I mention umbers and siennas, I mean the real umbers and siennas, not approximations of them sold with the same names, as is the practice of some paint companies now in the naming of their paints.)

From a structural standpoint, adding a small amount of a stand oil/mineral spirits or stand oil/turpentine 50/50 mixture to oil paints shouldn’t compromise the resulting film strength. It’s what I used for many years.

The trick is to use no more medium or oil than is absolutely necessary. Linseed oil does not add as much gloss as stand oil does.

Safflower Oil

Virgil’s assessment:

Safflower oil is a semi-drying oil that has been found to sometimes re-liquify years after supposedly being dry, and that’s why I recommend avoiding it.

“… a new discovery from conservation scientist Jaap Boon indicates possible problems with safflower oil reliquifying and weeping after it was supposed to have been dry for many years, so in the updated edition of my book I have removed all references to safflower oil as a suitable binding oil for oil paints.”

There are problems when Safflower Oil is the only binder:
“It makes weaker paint films, requires driers to help it dry at a reasonable rate, and is implicated in some instances of the oil re-liquifying years after it was supposedly dry. This might be a problem with only one variety of safflower oil, as there are several, however the issue hasn’t yet been investigated in enough depth to provide a clear picture, so the surest way of avoiding trouble is to just stick with paints that are bound with linseed or walnut oil, which have hundreds of years of good results to recommend them.”

Safflower and Sunflower oils make weak paint films that are crack-prone when they’re no longer young. There is also the potential for the paints to re-liquify and bleed or run or just grow sticky years after they’re supposed to have been dry.

Walnut Oil

Virgil’s assessment:

There are no significant drawbacks to walnut oil, other than it being somewhat inferior to linseed oil in film strength. The handling characteristics are more “short” than linseed oil. See page 122 of my book for an explanation of long versus short oils.

In response to a Facebook member who wants to use walnut oil as a medium for underpainting, Virgil responds:

“Ideally, no medium at all should be added to paint for underpainting. Walnut oil is not a good choice for two reasons; one, it forms weaker paint films than linseed oil, and two, it dries more slowly than linseed oil. The most structurally solid underpainting would have the leanest paints as its basis: lead white in particular, for its leanness, its fast drying, and the fact that it forms the most durable paint films of all oil paints, and confers that quality to all the paints used in conjunction with it. Tint it with high tinting strength colors such as the Mars (synthetic iron oxide) colors, or pigments that are lean by their nature, or both, all bound with linseed oil. Walnut oil is all right for the final layer.”

A Facebook member asks: “From the standpoint of constructing an oil painting soundly, is there a problem with using both walnut oil-based and linseed oil-based paints in various layers (including linseed over walnut) within the same painting?”  Virgil replies: “On a rigid support, it isn’t as critical an issue as it would be on stretched canvas, but ideally walnut oil paints should be introduced only near the end of the process, after the bulk of the painting has been done with linseed oil paints, if you’re using a multi-layer technique.”

Poppy Oil

Virgil’s assessment: 

Poppy oil is a slow drying oil that produces structurally inferior paint films compared to linseed oil.

Poppy oil paint films typically crack much worse than linseed oil paint films, in patterns unique to poppy oil. The long-term yellowing of linseed, poppy, and walnut oil isn’t significantly different unless the painting is deprived of light, in which case linseed yellows more, but the yellowing reverses when the painting is re-exposed to light.

Poppy oil dries slowly but makes weak paint films that are crack-prone when they’re no longer young.

Clove Oil

What it is: “Clove oil is a pale yellow to dark brown oil derived from the clove tree cultivated in the Moluccas or Spice Islands (now Maluka of Indonesia), Madagascar, Zanzibar, and the Philippines. The principal component of clove oil is eugenol. It is very slightly soluble in water and soluble in organic solvents. It has a spicy aroma and the taste of cloves. It is used in perfumes, flavorings, essential oils, and dentistry (as a local antiseptic and analgesic).” (Source: Natural Pigments)

Virgil’s assessment: 

To add clove oil to oil paints will slow the drying, but the trade-off is the risk of compromising the longevity of the painting; thus, I recommend not using it for any purpose other than treating a toothache, flavoring food, or making your house smell good.

While paintings to which clove oil has been added to the paints to slow the drying seem all right for perhaps several decades, the problem caused by it only becomes known when the varnish needs to be removed in restoration, and it’s discovered that the paint film underneath the varnish is weak and susceptible to attack by the solvents needed to remove the aged varnish. The result is an increased likelihood that paint will come off with the varnish.

Clove oil weakens oil paint films. Conservators have reported that paint to which clove oil was added comes off too easily with solvents commonly employed in restoration for removing darkened varnish, which is almost always necessary before other restoration procedures can be performed. That is why I recommend not using it.

Other Research Notes: 

MITRA: Extensive use/addition of clove oil makes the oil paint significantly more sensitive and it becomes impossible to remove and/or reduce a darkened/hazy/yellowed surface coating from the original paint layers without potentially causing irreversible damage.

Natural Pigments warning: “Although some artists recommend adding a drop of clove oil per inch of paint squeezed from the tube, we do not recommend its use in this manner. Although some claim that it darkens paint, using clove oil as a drying retarder in oil paint is greatly discouraged as its addition tends to weaken the dried paint film substantially.”

Balsams, including Turpentine

Virgil’s assessment:

I stopped using mediums with balsams in them many years ago, except for signing my name to my paintings. There was never more than a very small amount of any balsam in my “long” medium anyway, the main ingredient being stand oil or sun-thickened linseed oil.


Virgil’s assessment:
In response to a question about using wax to create an imposto, Virgil wrote:

Wax as an ingredient in oil paints is best limited to a very small percentage as a stabilizer, if it’s used at all. Its presence in an oil paint film renders it more easily resoluble in solvents normally used in cleaning old oil paintings. Wax becomes brittle and crack-prone in cold temperatures, and melts in higher temperatures. For those reasons, it’s not suitable for use as an impasto medium unless we don’t care about the future condition of our paintings. Impasto is best achieved with lean oil paint with no wax added. White lead is the ideal base ingredient for impasto. Its low tinting strength allows it to be tinted with colors whose tinting strength is higher, and its density, flexibility, and durability are superior to all other oil paints. Rembrandt’s impastos have been found to have ground glass in them, but no wax. Earlier writings stating that he used wax were subsequently shown to have been mistaken after the paintings were analyzed by modern scientific test methods.

Re: Cold Wax as a medium: See page 136. Cold wax softens the paint. It has solvent in it. Brushing on an oil varnish will risk dissolving the paint layer where it is used.

Wax is best left out of oil paintings that are intended to last. It renders oil paint films susceptible to paint loss in restoration procedures, and to softening and possibly running if/when subjected to high temperatures. Used as a stabilizer, beeswax is acceptable, but only at 2% or less. Beyond that, wax should be considered detrimental to the longevity of oil paintings.

A comment from George O’Hanlon, Natural Pigments, on beeswax: “Although wax increases the viscosity of oil paint, it does so by sacrificing its resistance to solvents and makes oil paint more brittle. Cold wax is not a recommended practice for use with oil paint, due to the solubility of wax as opposed to the decreased solubility of oil paint. We recommend using wax very sparingly in oil paint to avoid these issues.”

Damar or Dammar Resin

Virgil’s assessment:

Damar grows brittle with age, turns yellow and then brown irreversibly, and renders oil paint films to which it has been added susceptible to accidental removal when the painting is cleaned in restoration.

If you care how long your paintings will last, you should rethink the idea of using mastic and/or damar resin in mediums to be used with oil paints or as final varnishes for them. These are problematic substances in oil paintings.

Technical Reference:

Mastic Resin and Maroger

Virgil’s assessment:

In response to questions about Maroger as a medium:

“Old Masters Maroger” earns my disdain by its misleading name, for one thing, because Jacques Maroger’s claims that it was used by the Old Masters is unfounded and most likely untrue, if we can trust modern science’s test results of paint samples from actual Old Master paintings more than Maroger’s guesswork from 50-60-70 years ago before those test methods existed. But Mr. Maroger was a very persuasive writer even when he was wrong, and a good painter, so there were and are many people who have been taken in by his spiel and the romantic notion that he had discovered the Lost Secret of the Old Masters, and some of those disciples of his became very good artists themselves and went on to teach others what JM had taught them. Frank Mason, Ann Didusch Schuler, and Joseph Sheppard are three who come to mind right now. David Leffel learned from Frank Mason before Mason realized he had been misled, by which time David had been painting and teaching what he had been taught for decades.

The medium itself contains mastic resin, a substance that turns yellow and then brown irreversibly, adds brittleness to paint films to which it is added, and renders those paint films vulnerable to solvents commonly employed in cleaning old oil paintings.
Mastic resin is mixed with black oil and turpentine to form a soft jelly. Black oil is linseed oil cooked at high temperatures with lead monoxide (litharge) or white lead pigment in the presence of air, thus it is pre-oxidized. Oxidation is the process of breaking down, of deteriorating, decomposing, weakening. The jelly is soft, and thus softens stiff oil paints that are too thick for optimal control under the brush, and that is a big factor in its ongoing popularity.
However, it has a long history of causing defects in oil paintings made with it, going back to the mid-18th century when Joshua Reynolds used it in his attempts to create Rembrandt-like effects in his paintings, which paintings are notorious in conservation circles for the problems they present to restorers and conservators.
I consider it very unfortunate that so many truly fine painters continue to use Maroger’s concoctions, in light of the problems it brings to their paintings. A number of these painters are friends of mine, and their paintings are excellent; so good that the thought of them deteriorating prematurely is a very upsetting prospect.  The fact is that everything that can be done with that medium can be done every bit as well without it, so there is no good reason to fool around with it.
I don’t recommend using any of Maroger’s concoctions on any painting intended to last as long as fine art oil paintings have lasted historically. Despite the misleading names on these mediums, neither wax nor mastic resin have been found in paintings from the 17th century or earlier, according to the conservation scientists who have analyzed them.

Copal Resin

Virgil’s assessment:

I’ve tried copal and amber mediums in the past, and found they were more compatible with painting on smooth panels in small sizes using soft-hair brushes, rather than larger paintings on canvas that has more texture to it. For the latter, these resins add too much gloss to the paint, which makes large paintings harder to read if they’re not lit from high above. I found that in order to avoid extremes in variations of surface gloss, I had to eliminate resins and polymerized oils from my paints to reduce the glossiness of those passages, and umbers from my palette to eliminate or reduce the dry-appearing patches commonly referred to as “sinking in.” My medium now is just water-washed, sun-bleached linseed oil, and not very much of that.

There is also darkening of the natural resins to consider, as it’s irreversible, and embrittlement, which would be less of a concern on a rigid panel than on stretched canvas.

I glue my canvas to a panel, but I don’t see a great need for copal or amber resins in my own work. I do add a bit of copal to the paint with which I sign my signature, because I like “long” paint for that, but otherwise I don’t bother with it.

Amber Resin

Virgil’s assessment: 

A Facebook member asks about a Amber medium on the market made by Blockx and promoted as one of the secrets of the Old Masters.  Virgil responds: Blockx’s information that forms the basis of their assertions is over 100 years old. We might entertain a degree of doubt when a company that wants to sell us a product tells us how wonderful it is. They might not be totally objective in their evaluation. Just a thought.

My concern with the Blockx amber medium is that it’s made with poppy oil. Poppy oil is a slow drying oil that produces structurally inferior paint films compared to linseed oil.

George O’Hanlon of Natural Pigments adds: “If you read my article on resins I do not recommend resins in oil painting but, if resins are used, hard resins such as alkyd, copal and possibly amber are better than soft resins such as dammar and mastic. However the main issue with resins is their differences in solubility and drying mechanism. They complex the paint film and should only be used sparingly, if at all. There is little evidence of the use by the old masters and there is even less knowledge of how it performs with age.”


Liquin is an alkyd medium made by Winsor & Newton that contains both alkyds (modified oils that dry faster than regular vegetable oils) and solvents.  It should not be used as a varnish.

Virgil’s assessment:  Liquin is a good medium if you want fast drying and your working space is well ventilated. It contains a volatile solvent, so there are vapors to consider. It isn’t intended to be an impasto medium for thick painting…

I see no problems in it as far as the structural soundness of the painting itself is concerned…  the solvent in the Liquin give off vapors that can be harmful to breathe, however, which might or might not bother you right now, but could cause health consequences at some point after longer exposure. If you’re painting outdoors it shouldn’t be a problem, but otherwise make sure there is good ventilation in your workspace, and do everything you can to keep the solvent vapors there down to as low a level as possible.

My top recommendation is to use no solvents or mediums that contain solvent. If the paint is too thick for good control, as it is with some brands, a very small addition of linseed oil mixed into the paint on the palette thoroughly with a palette knife should correct that problem, and if it doesn’t, a change of brands might be in order.

In response to a Facebook group user’s concerns about older bottles of Liquin appearing too dark, Virgil writes: “Liquin doesn’t darken in paint any more than linseed oil does, unless too much is added to the paint, but the darkening of Liquin and also of linseed oil is not enough to cause any legitimate concern. The yellowing/darkening seen in short-term aging reverses with exposure to light. The secondary yellowing that occurs after long aging is so slight as to be negligible, as long as the ratio of medium to pigment is not excessive.”… “It’s not enough to affect the appearance of the paint as long as the mixture is 15% or less Liquin/85% or more paint. 10% Liquin/90% oil paint would be still better.”

Used without paint in it, as a varnish, Liquin will be very difficult to remove if or when it ever darkens (which it will) or develops some other problem that interferes with the viewing of the painting or alters its appearance in some way. The solvents that will be needed to remove it will take the paint off too. Winsor & Newton advise against using Liquin in that way.

Solvents: General Comments

All oil painting solvents are inhalation hazards, no matter how ‘non-toxic’ they claim to be. Use with ventilation.

Virgil’s Position on Solvents: 

It really isn’t necessary to use turpentine or any other solvent when painting in oils. Solvents weaken the binding power of oil paints, and the vapors are harmful to breathe. Your painting and your health will fare better if you adapt to solvent-free oil painting.

I’m not advocating for solvents of any kind. I’m advocating painting without solvents, in the interest of the longevity of the painting and the health of the painter and whoever else shares the air in the painting space.

I’ll reiterate that it is not necessary to use turpentine, or mineral spirits, or spike oil, or citrus peel solvent or any kind of solvent when painting with oil paints.


Two good reasons not to thin oil paints with solvent:
1) you and all who share your air space are spared the potential harm to your/their health that inhalation of solvent vapors could cause, and
2) solvents mixed with oil paint weaken the binding power of the oil binder, thus increasing the likelihood of problems developing in your paintings at some point in the future.

If you find your paints too thick for good control, mix in a tiny drop of linseed or walnut oil to each pile of paint on the palette, and mix it in thoroughly with a palette knife. The longer you work it with the knife, the softer it will become, so not much oil is needed.

To clean brushes while painting, have two jars of linseed, walnut, or safflower oil ready for that purpose; wipe the brush on a rag or paper towel, rinse in the first jar, wipe again, rinse in jar #2, wipe again, and the brush is ready for a new color. Same procedure after the session is over, then wash each brush with soap and water. This is in my book, by the way.

Odorless Mineral Spirits (OMS)

Virgil’s assessment:

Gamsol is the mildest kind of odorless mineral spirits.

You can use it to clean an oil painting prior to varnishing by gently wiping the surface of the painting with a soft cloth very lightly moistened with it. There should be no adverse effects from this procedure if the painting has had at least six months to cure. This is not theory or guessing on my part, but is based on actual experience doing it at the suggestion of Ross Merrill when he was Chief of Conservation at the National Gallery in Washington DC.

Gum Turpentine

The solvent that speeds drying is gum turpentine. Mineral spirits does not, nor does spike lavender, which is a slower drying solvent. However, they are all best left out, because they can compromise adhesive strength and film strength of the paints to which they are added.

What effect does using gum turpentine have on oil paint?

Virgil’s assessment:
Gum turpentine will thin oil paints, and is used in that way by many painters; however, thinning oil paints too far will weaken the adhesive strength and potentially compromise the adhesion of the paint to the ground or previous layers. It can also remove dried oil paint that is not well cured. Mixed with oil paints while painting, it’s reported to increase the tendency to yellow with age. It does speed up the drying slightly.

In the context of dissolving varnishes:  gum turpentine dissolves Damar varnish.

If you feel you have too thick a coat of varnish on your painting, you can thin it out by brushing over it with the appropriate solvent, and then again with dry brushes while it’s wet. Which solvent is the right one to use would depend on what resin the varnish has in it. If it’s damar, then gum turpentine would be the right solvent. If it’s an acrylic MSA varnish , mineral spirits is the correct solvent; if regalrez (Gamvar, etc.), then odorless mineral spirits will do it.

Another reply to a reader who wants to remove Damar varnish from an old painting before touching it up again:

“Gum turpentine is the common solvent used in damar varnish, and is the weakest solvent that will remove it. Odorless mineral spirits will not likely remove all of the damar… After the varnish is off, allow several days for the solvents to leave the paint film before you begin applying new oil paint.”

Spike Lavender Oil


Spike lavender oil is a solvent, not an oil; it thins oil paints, mediums and varnishes in the same way that odorless mineral spirits (OMS) would do.  It is a strong solvent that dries slowly.

Beware of art suppliers who claim that spike lavender oil is safer than other solvents or even non-toxic. The effects of inhaling spike oil for artists are unknown; precautions should be taken. Use spike lavender oil with adequate ventilation and/or protective gear.

Keep in mind that it is not necessary to use any solvent in order to paint well with oils.

Virgil’s assessment:

Everything you need to know about lavender and spike oils: Natural Pigments article.
The bottom of the article discusses the historic use of spike oil in art.

Virgil supports the advice of expert conservator,  Kristin DeGhetaldi, PhD, who worked in the Conservation Department of the National Gallery in Washington DC before starting the MITRA website. She has written: “Spike Lavender oil contains a number of terpenes and other materials with the same health code rating as OMS. Spike lavender oil is much slower evaporating so it is more likely to bite into underlayers….the only reason to use it instead of OMS is if one likes the smell…if you are however trying to dilute things like stand oil or resinous medium (which is not particularly encouraged) some grades of OMS will not work as well compared to MS, Turp, and/or spike lavender. If you are looking for something that is less aggressive at thinning oil paint OMS is probably the best option.”

More from Virgil:  There are some products being marketed as lavender spike oil for use in oil painting that have other ingredients in them that might not actually qualify as non-toxic, so I would not be too eager to accept salesmen’s claims of wonderful properties, health safety in particular. We should not assume that things that have not been tested for their health effects and declared toxic are therefore necessarily non-toxic. I would expect any volatile solvent’s vapors to pose health risks.

In any case, it isn’t necessary to use any solvent in order to paint well with oils.

Another resource: Tad Spurgeon’s article reviewing the available products on the market.

Virgil also recommends avoid using varnish that contains lavender oil. (Example: Chelsea Studio Classic Lavender Damar Varnish). The lavender oil is a strong solvent that dries slowly. There is the chance that it could eat into the paint, and possibly smear it or remove some of it unless the paint is well cured before the varnish is brushed on. Wait at least one year after the last brushstroke is dry before applying it.