Fortunately there are several ways you can slow down acrylics' drying time. Below are a few of the methods that can buy you some extra time:
Use Open MediumThis acrylic medium, made by Golden, is an economical way to extend the working time of "regular" fluid acrylics, soft body acrylics and heavy body acrylics. Add a drop of two of this medium to each color on your palette, mix it in with a palette knife, and you'll gain anywhere from 20 minutes to an hour or more of additional working time. Exactly how long depends on the amount of paint you've put out on your palette, atmospheric conditions, and the viscosity of the type of acrylic paint you're using.
Use a Wet PaletteSeveral manufacturers offer specially made palettes featuring sponges in them that can keep acrylics workable for quite a long time if the sponge is not allowed to dry out. My Sta-Wet palette usually keeps them viable for about two weeks with the lid on. The only drawback I've found is that our humidity levels here in Georgia are often high enough to encourage the formation of mold in the palette's closed environment. If you store your palette away and forget there's paint in it, be prepared for a less-than-fragrant experience when you get around to using it again.
Use Open AcrylicsA few years ago, Golden Paints introduced a line of acrylic paints and mediums designed to stay workable much longer than conventional acrylics. Open acrylics are great. If you're just starting to build a good selection of colors, I'd recommend buying Open products exclusively. If you already own tubes, jars and bottles of regular fluid, soft body and heavy body products, don't despair. You can mix Open acrylics with most brands of acrylic paint and those paints will stay wet noticeably longer. (There are a few brands which are not compatible with Open Acrylics, so be sure to read labels carefully. If necessary, call the manufacturer and ask.)
Use RetarderRetarder is somewhat like Open Medium in that it is a liquid that's added to the paint in small doses. However, retarder is much trickier to use: you must follow the directions on the label to the letter. Mix too much retarder in with your paint and your painting can remain wet or tacky for years.
Use WaterWater is what we used back in the "olden days" to keep acrylic paints viable. A quick spritz of the paints on the palette does work, but over time, repeated spritzings will overly dilute the paint. If you use water, use it sparingly.
Use Gels and MediumsSome acrylic gels, such as Soft Gel or Regular Gel, will keep paint from drying too fast both on the palette and on the canvas. Most matte medium, gloss medium, string gel, self-leveling gel and other such products can also extend acrylics' working time.
Try any of these methods as your budget and inclinations allow. I tend to rely heavily on my bottle of Open Medium to keep my paints wet because it's convenient to use and a little goes a long way. You may find that one of the other methods works better for you. Try them all; you're sure to find one that's compatible with your working style and budget.
Note: see my post of 11/22/15 for one more way to keep your acrylics wet longer.
©2015 Lynn Edwards