I don’t consider my acrylic paintings finished until they are varnished. I find that acrylic paint stays soft and sort of sticky. This means that the surface gets easily damaged because it sticks to anything and bits of paint can even come off. Applying a layer of polymer varnish (acrylic solution varnish) seals it with a hard, non-sticky layer.
Don’t forget that acrylic paint takes longer to cure than you might think. It dries to the touch quickly, but the curing is done by polymerisation like oil paint and it can take up to a month for an acrylic painting to be dry enough to varnish. If the paint is thick or you used a retarder or any of the slow drying acrylics it could be longer. I have used thick layers of gel that have taken 3 months to go from cloudy to all the way clear. Oil paintings usually take 6 months to a year to be dry enough to varnish, but it can be even longer if the paint is very thick or the environment is very humid.
Depending on the painting and the effect I am after I use gloss, matte or a combination which makes satin. Gloss varnish can look really nice, like the paint is still wet, really rich and shiny. But there is a huge drawback, it reflects like a mirror and the picture is actually hard to see very well. It also highlights all the ridges and bumps and any unintentional texture and that can distract from your intentional marks. A matte surface can eliminate all of this but it can also make things appear a bit paler, as the matting agent is a white powder, and you need to be very smooth and even with your application or it can look streaky. Golden recommends a layer of clear acrylic as an isolation coat always but especially under a matte varnish to prevent the white layer showing as much. Sometimes a mixture to make satin is the easiest choice to get more of the good points and less of the bad. But really, you should decide depending on the mood and meaning of the painting and what effect you are after.
Varnishing is a skill. I have gotten better at it over the years. A good soft brush (I like this one) and a dust-free area are essential. Take care of your varnishing brush, it needs to stay soft. A few thin coats is better than a thick one, it should dry quickly. It helps if you position a light so the light rakes across the surface and you can see the shiny parts you have done so you don’t miss a spot. (Using raking light, shining from a side angle, to examine your painting is helpful as a final finishing step as well.)
Varnishing needs to be done properly or it can ruin your painting. You can learn to do it yourself, maybe think of learning how as part of learning to paint. Take the time to try different varnishes and different applications and learn what you like best. And don’t take it for granted. I have heard that some artists regularly have a varnish specialist do the varnishing of their paintings. I think it is nice to be able to do it yourself, and that way you also have control over the appearance.
When the varnish is touch dry you can lean the painting up against a wall with the paint side facing the wall, with just the edge of the painting touching the wall, to finish drying. This will allow any dust to fall on the back of the painting as the face of the painting is protected. (This is a good position for drying paintings in general.)
Instead of thinking of varnishing as an afterthought, it is good to remember that the varnish is part of the structure of your painting. The longevity of this artwork you have put so much time, effort and thought into depends on everything from the support to the varnish.
Artist’s varnishes should be removable. But that doesn’t mean they will be easy to remove. Varnishes for oil paintings are removed with the same solvent that will remove the paint. The varnish should only be removed by a professional restorer using a cotton bud. Slowly, slowly, you stop when the cotton bud has colour on it. You may be able to clean an old, dirty varnish by removing a thin layer of it with mineral spirits, without removing the whole thing. But again, it is a proper job, not a quick thing. Be respectful of the painting. For acrylic paintings Golden polymer varnish can be removed with household ammonia which will not dissolve the paint. But it still changes the surface and the one time I had to remove it because I made a mistake with the varnish, the surface looked dull and roughed up a bit. I probably should have been more gentle.
I also didn’t use an isolation coat back then. An isolation coat of clear acrylic polymer acts as a protective barrier and is recommended to go between the painting and the varnish. Golden has a good video showing how to do this.
Because acrylic paintings are so soft and sticky they attract dust as well. Museum conservators are finding it difficult to clean the acrylic paintings from the 60’s whose porous, sticky surfaces have trapped a lot of dust. A coat of varnish will reduce the tackiness of the surface so less dust and dirt will stick, it will make it easier to wipe clean over the years, and in the far future some conservator may thank you, when all they need to do is carefully remove your varnish and the painting will be like new! Here is a great technical article on acrylic painting conservation.
Some artists also like that a coat of varnish evens out the level of sheen. Each pigment needs a slightly different proportion of ingredients and this causes differing levels of shine for different colours of paint. If you use more than one brand in a painting that can make even a bigger difference in the shine of each colour as some makes are more matte than others. If you want the effect you can even put matte varnish on part and gloss on other parts of the painting.
All varnish has a glossy base, matte varnish (and satin to a lesser degree) has matting agents in it, either a dry particulate matter or a wax. In both acrylic and oil paint varnishes the matting agent usually has to be redistributed into the varnish by agitation, as the matting agent settles to the bottom. It is easier to see this glob of matting agent settled in the bottom if the container is clear. So- stirring or shaking without getting too many bubbles in, then letting any bubbles settle, a gentle application to prevent forming foam on the surface of the painting, and testing a corner of the painting first are all good rules. If you are a first time varnisher, do a few of your less important paintings first to get a feel for it. (As far as the agitation goes: maybe mail order helps, cos it gets bounced around on its way to you!)
Some artists use acrylic varnish that is made with a solvent rather than water, like Golden MSA (mineral spirit acrylic) varnishes (acrylic dispersion varnish). These are especially useful if the artwork needs protection from the elements, like an outdoor wall mural, because it is extra tough. The MSA also comes is an aerosol can (spray varnish is easier to apply evenly). Spray varnish is very dangerous to your health unless you have a spray booth to extract all the fumes from the room. The MSA will also work to varnish oil paintings. Both types of Golden varnishes, the water-soluble polymer varnish and the solvent-based MSA contain UV filters.
Much of the same advice applies to varnishing an oil painting. Wait until it is bone dry, usually 6 months to a year. 2 or 3 thin even coats are better than a puddle. Practice first to get good at coating evenly and feathering in. Use a modern synthetic resin varnish for oil paintings, the old natural resins like damar varnishes usually yellow and deteriorate and most people have given up on them. The surface of an oil painting is more durable than an acrylic painting but a varnish is still a good idea for protection. If your aesthetic decision is to not varnish because you don’t like the glossy surface you can get a matte varnish for oil.