I've been wanting to address this issue for some time, because I've received so many emails from people who saw my videos and want to know how I keep my paints wet. So, here is the scoop...
I store my paints (Golden Heavy Body) in a plastic craft bead box, or bead organizer. The one I use is divided into 18 compartments, the brand name is "Darice". I bought it at Michaels or AC Moore. It is a type of plastic that is sort of flexible and has some give to it (high density polyethylene). I don't buy the type made from hard plastic which is as rigid as glass, and snaps if you try to bend it. Reason being, the acrylic paint adheres to that type of rigid plastic and does not peel off when dry. BTW, the bead box is not air tight.
The reason my paints don't dry out, I am assuming, is that there is a LOT of paint in each compartment, and the amount of paint stored is a pretty thick depth-wise. Acrylics dry when the water in the paint evaporates, and due to the thickness of the paint in each compartment, it takes a lot of time for the water to work its way out. Plus I spritz all the paints while I am working. I may not have done that in the video, but in between filming segments, I spritzed. Also, I put the whole container in a ziplock 2 gallon bag with a moist paper towel. My paints last for months and months without drying out. Its not a perfect solution, as eventually they seem to get a tad dry, but it works for me. I don't like setting out blobs of paint each time I have some time to paint, this allows me to flip the lid and start immediately. If you put tiny little blobs in the paint compartments, I can guarantee they will dry out fast.
If you want acrylic paints that dry 10 times slower, try "OPEN Paints", also by Golden. They are a slightly lower viscosity (not as thick). You can mix the regular heavy body paints with the OPEN paints too - they are perfectly compatible. If you mix in a 50/50 proportion, your paints will dry 5 times slower than the straight heavy body colors. You can also use the slow drying OPEN Medium, which, when mixed with either the OPEN or heavy body paints, helps increase the flow of the paint off your brush, and extends the dry time. You can also put a drop or two of OPEN Medium on each of your colors, whether you are working with blobs on your palette or in the compartments.
Many people use water as the medium with acrylics, not realizing that they could be using a medium such as "Polymer Medium". When I want my paints to dry faster, I use water as the medium. When I want them to dry slower, I use a medium. Polymer Medium is glossy, Matte Medium is matte. OPEN is the slowest drying medium, and is available in both sheens. Again, all OPEN products are designed to work with the faster drying acrylics too.
Another tip: You can use OPEN Titanium White in your mixures (rather than regular heavy body Titanium) to extend the dry time in any mixtures that have white in them.
I hope that helps! I hope it is also obvious why I did not answer each email individually :-)
On another note, I just got back from teaching a fabulous workshop in upstate New York. There were many break throughs - and I saw them happen! So many "ah ha! moments"... which makes me so happy. I had a great crew of gals and guy, and was really inspired by the amazing work everyone did.
I'm looking forward to my next workshop in Tuscany in September. BTW it is open to all media --not just acrylics, as the fundamentals, value, color schemes, composition, etc, apply to all. Hope you can join us!