When it comes to varnishing your paintings, there are 2 main types of varnishes you need to know about, Intermediate Varnishes, and Final Varnishes.
'Intermediate Varnish', often referred to as 'Retouch Varnish', is a varnish that is meant to add luster to your painting in between coats. This allows you to better assess colors that may have dried matte, and then work back on top of them. An intermediate varnish forms a semi-permeable layer, which allows your paint to continue drying beneath it. And although you can use an intermediate varnish when you're completely finished with your painting, it's affects will fade rather quickly, likely in a year or two.
The other kind of varnish is what is called a 'Final Varnish'. This kind of varnish is meant to be applied after your painting is completely dry. It not only adds luster back to your colors, but protects the surface of your painting from damage, harmful UV rays, dust and moisture. Unlike an intermediate varnish, a final varnish provides a much more permanent and fairly impermeable barrier, which means you need to be sure your painting is completely dry before applying it.
A final varnish is designed to be easily removed with mineral spirits, so that when a painting inevitable shows signs of yellowing of cracking, conservators can easily remove that varnish and apply a fresh one.
You could achieve similar results by using a coat of medium (such as linseed oil or Galkyd), but this is not advisable. A medium will bond to paint layers, and is therefore not removable. So if the surface of your painting cracks, it's difficult to remedy.
Think of a Final Varnish a bit like waxing your car. It adds shine and protects the surface, but isn't meant to last forever. You will eventually want to reapply it. However, unlike a retouch varnish, that reapplication might not be needed for 50 years or more.
There are all sorts of final varnishes out there. One of the most common being Dammar Varnish. Dammar Varnish has been used for centuries, but is made from natural resins, which means it yellows significantly, is very brittle and is difficult for conservators to remove. Dammar varnish is what adds that antique yellow tinge you see in many classical paintings.
Nowadays, most conservators recommend using a synthetic varnish. They yellow less, are more durable, and easier to remove.
(Above: A painting finished with several coats of Galkyd can create a glass-like sheen over your work, though it is non-removeable)
Still, there are numerous options out there to choose from.
For years, I used a very particular brand of final varnish that I really liked. It provided a sheen that I felt was just right for my work... not too glossy, not too matte. It also had a consistency I really liked, that was thin enough to apply easily, but viscous enough to even the physical surface of the painting. Unfortunately, that brand has been discontinued, so I had to find a newer alternative.
In a quest to find one I really liked, I decided to test just about every varnish I could get my hands on. I tried varnishes in both spray form, and the more traditional brush-on form.
I've now tested 19 in all, and decided to post my results here for you. It is worth mentioning that my preferences here have a lot to do with personal taste, and the particular surface I work on. What works well on illustration board may not work as well on canvas for instance. You may find that a varnish I hate works really well for you.
1. W&N Artist's Picture Varnish Satin:
When I was first starting out, this was my varnish of choice. Unfortunately, it tends to bead up on oily areas, and does not take a second coat very well. If you paint with lots of glazes or thick mediums, this one is not ideal.
2. W&N Artist's Picture Varnish Gloss:
Same as above, but much too glossy for my tastes. The gloss heightens the appearance of surface imperfections.
3. W&N Artist's Picture Varnish Matte:
In general, I do not like Matte Varnishes. They reduce sheen, which is precisely the opposite result of what I am trying to achieve with a varnish. However, I have seen matte varnishes work very well on more fine-art looking works.
4. W&N Artist's Retouch Gloss:
Again, similar to the sprays above, often providing a speckled, uneven surface.
5. Krylon Conservation Varnish Gloss:
Not worth the money I spent on it. Definitely my least favorite of the sprays I've tried. Goes on very unevenly. Beads up in oily areas, and puddles in others. (see video below)
6. Grumbacher Final Varnish Matte:
Very good if you like a matte surface.
7. Grumbacher Final Varnish Gloss:
Perfect for my needs. By far my favorite varnish and very easy to find at your local hobby shop.
8. Grumbacher Damar Varnish:
Similar to above, but contains natural Dammar resin which is not as archival. However, some people prefer still Dammar for it's high viscosity. The added thickness evens the surface a bit more than the non-dammar version.
9. Grumbacher Original Picture Varnish:
Pretty good, but I suspect it may contain Dammar, as it has already shown significant yellowing in just a few years.
10. Grumbacher Damar Retouch:
'Damar Retouch' sounds like an oxymoron to me. Dammar by it's nature is very enamel-like, and does not do what a good retouch should do. That said, this varnish has still impressed me, and has saved my butt more than once. If you work on an oil primed surface, things can get pretty slick pretty quick, making it difficult for new brushstrokes to adhere to previous layers. This varnish dries somewhat 'tacky', which helps the surface grab the paint better. Still, it's final varnish capabilities are less than ideal, which is to be expected from a retouch.
11. W&N Artists Gloss Varnish:
Easy to find at local hobby shops, and pretty decent. However, I found it's binding capabilities to be less than impressive. It removes TOO easily in my opinion. You can easily scratch this varnish off the surface.
12. W&N Artists Matte Varnish:
Same as above, but with a cloudy matte finish. Scuffs easily.
13. W&N Dammar:
Dammar is a staple in the industry for a reason. Even though it yellows and cracks, it's honey-like consistency is really nice and hard to beat for those who like a thick varnish.
14. W&N Artist's Retouch:
If I need to use an intermediate varnish, this is my go-to. However, it is not adequate to use as a final varnish, It's affects will fade noticeably in just a few years and will definitely require a re-application.
15. Weber Synvar:
Undoubtedly the WORST varnish I have ever used. Unbelievably poor adhesion, which is a shame because the luster it provides is quite nice. (see video below).
16. Gamblin Galkyd:
This is technically a medium, which means it will bond to your paint film on a chemical level, and is not removable. It will also yellow slightly with age. However, if you don't care about that, and you want something that looks like a literal layer of glass over your painting, this works surprisingly well. Just be aware that a thick, hard varnish will eventually crack on a flexible surface like stretched canvas. This works best on rigid surfaces like wood. I applied multiple coats, with a light sanding between coats. (see video above)
17. W&N Conserv-Art Gloss:
This is still my absolute favorite varnish. Just the right sheen, and a honey-like consistency that settles into cracks slowly and creates a smooth surface. Unfortunately, it has been discontinued and has become increasingly difficult to find in the US.
(EDIT: Apparently, Conserv-Art has simply been repackaged and is now called "W&N Artist's Varnish")
18. Gamblin Gamvar Gloss:
This is my new go-to for brush-on varnishes since the one above is no longer available. It is not as thick as the Conserv-art, so brushstrokes show more, but thus far, it is the best archival replacement I have found. You can dilute it with Gamsol, or add a cold-wax medium to it to control the level of sheen you desire.
19. Gamblin Gamvar Matte:
The same as above, but with a pre-mixed matte finish. Too matte for my tastes.
THE WINNERS AND LOSERS
The Best Spray-On and Best Overall
Surprisingly, my new favorite varnish is a spray varnish. I never used to like spray varnishes because I felt they never really settled into the crevices of a painting very well, but Grumbacher's Final Varnish Spray (Gloss) has changed my mind!
It coats really evenly, has great adhesion (even in oily areas), and takes multiple coats very well. I find it also evens the sheen of my paintings. Areas that are matte become glossy, but more unexpectedly, areas that I think are too glossy actually get matted slightly as a result of the fine aerosol texture.
I tend to apply this varnish in one moderate coat, going just barely heavier than I need, and then allow it to settle in. If needed, I can give it a second light misting, which binds to the first coat very well and does not matte out the surface as many other aerosols tend do.
The Worst Spray-On
The worst spray varnish I tried was Krylon's Conservation Varnish (Gloss). It beaded really badly in oily areas, and puddled up in the less oily areas, resulting in an incredibly uneven surface. Multiple coats did little to remedy this.
The Best Brush-On
As for Brush-on Varnishes, I'm still not as in love with any as much as the discontinued 'Conserv-Art', but if I have to choose a replacement, Gamvar does a pretty darn good job. If you're looking for something versatile, with a strong emphasis on safety and archival ability, this is a good choice.
The only downside to Gamvar for me is it's thinness. It simply makes your painting look wet again, but doesn't even out the surface any. Many may like it for just this reason, but I personally prefer a little more viscosity and a more enamel-like finish. Galkyd Lite actually does a better job of this in my opinion, but is non-removable.
The Worst Brush-On
By far, the worse varnish I tried was Weber's Synvar. A final varnish is not supposed to chemically bond to paint film beneath it. But this varnish sticks to the surface so poorly, that just the moisture (or maybe the oils) in your hands will cause the varnish to peel off your painting like a bad sunburn. If you hold your painting in one spot for more than a minute, varnish will stick to your fingers instead of the painting. I would avoid this one at all costs. I found that the W&N brush-on varnishes do this to some extent too, but to a far lesser degree.