Anyone who's done any research quickly learns that there are some structural problems that can happen when oil paintings are varnished before the three to six month drying time.
Of course, as artists that sell work, this is somewhat problematic. We don't necessarily like storing pieces for that long. Fortunately, times change. New studies and experiments happen. While I'm not a chemist or materials expert, I am a researcher-type and I'll share some art "street talk" about it here.
First of all, a huge factor in deciding how long you should wait, and what varnish type you should choose, is how your painting is materially constructed. If you are oil painting with thick layers (I don't) or heavy impasto, you should wait longer than the painter following fat over lean principles, who builds paintings using quick-drying earth colors and underpainting whites to add texture (what I do). If you are painting using truly old masters' mediums and methods, you should stick with the rules and wait at least three months. Some of the old mediums are very viscous and contain slow-drying oils.
The current favorite varnish seems to be GamVar. If your painting is dry to the touch, you may use it. Personally, I'd be very careful about this. If your paint layers are thick or contain slow drying oils, I'd dilute the GamVar with 20% GamSol (compatible brand solvent) and apply it as a retouch varnish. If you need to show and sell a piece fast- this is what I do.
Alternatively, I'm intrigued by a post I read this week by Kyle Surges, https://www.nitpickyartist.com/wax-as-a-varnish.html. I like the look of wax, and plan to experiment with his suggestion. Check out his post. He no longer likes to varnish his paintings, but is concerned with protecting the surfaces and wants the colors saturated. If you don't understand what that means, it means that when colors dry, they tend to get a slight light blue have over them. The darker, warm passages don't look like they did when the paint was fresh.
Damar Varnish is a traditional, high gloss varnish. It is often used diluted in old mediums. A modern understanding of varnishes is that they should not bond so much with the paint layers that they strip the color when the varnish is removed.
Below, I have quoted a blog post from Ron Sanders. He had a phone conversation with Catherine Metzger of the National Gallery, Washington D.C., and these are his take-aways from the conversation- not direct quotes of Ms. Metzger. See link below to go to Ron's site and read more. Also worth noting is that Gamblin, the maker of Gamvar and Gamsol, has a video tutorial on the proper way to apply their varnish. Link here.
As mentioned earlier, damar varnish can be difficult to remove from a painting without damage when aged for a long period. This becomes especially true when large amounts of damar have been used in the painting itself.
For safety sake, if one wishes to use damar in one's medium, then a synthetic varnish is advisable. Catherine highly recommended Gamblin's "GamVar" varnish. Other synthetics may still crosslink with the paint film underneath. But GamVar is a hydrocarbon based varnish that does not crosslink and removes easily with mineral spirits (which do not dissolve damar). Also, it does not yellow or crack.
Another benefit is that GamVar is flexible enough to be applied as soon as the painting is dry to the touch, instead of waiting 6 to 12 months. This means that artists can apply a full final varnish before rushing paintings out the door to galleries. Catherine says that no varnish seals out air from a painting, and that immediate varnishing will not hinder oxidation of the paint layers beneath.
GamVar is now available premixed in 5.4 and 2.0 oz. jars.
The gloss of GamVar is meant to be comparable to damar, but some say that it is shinier. To reduce gloss, add solvent (mineral spirits) or wax medium.
No resin protects against oxidation or atmospheric moisture. Its sole purposes are to protect against dirt and atmospheric pollution, and to even out the sheen of a painting, adding depth to color.
After our discussion I ordered a batch of GamVar to prepare a group of paintings for a show. The varnish flowed evenly over the paintings when applied and coverage was exceptional. I mixed the varnish 2 parts to one part Turpenoid and found the gloss a light sparkle and not at all excessive. I was able to apply it as soon as paintings were dry to the touch. And best of all, it dried over night, whereas damar varnish has often taken the better part of a week.http://www.sanders-studios.com/instruction/tutorials/historyanddefinitions/nationalgallery.html
In summary, when in doubt, make a diluted version of GamVar or try using the wax medium if you don't care for the high gloss look anyway. LIke Kyle Surges, I've had trouble with GamVar beading on paintings. I have overcome that by using a super soft cloth to wipe down the dry oil painting surface with a touch of clean GamSol. The little bit of solvent cuts the oil and prepares the surface for the varnish. i won't deny that the process does make me very nervous, which is why I'll experiment with the wax on an expendable painting.
I hope this helps! Let me know if you have a solid, professional opinion in contradiction or support of these methods, I'm listening.