Ah, this subject is very familiar to me. Specifically Accu Paints. I still have my stash and some of my paints go back to the early 1990s! Here are the secrets of my success.
I noticed that either the plastic bottles or the plastic lid seal are permeable to the solvent and allow it to evaporate over the years. My solution was simple: transfer the contents of all the plastic bottles to the standard glass 1oz. Floquil paint bottles. Problem solved! Well, almost. I also added aluminum seals over the standard cardboard seal in the Floquil caps.
I made the seals using the thicker aluminum foil used in disposable baking pans. I took one of the cardboard seals and used it as a template to draw circles in the aluminum sheet and then cut out the seals.
These 2 things took care of evaporation. How do I know? Another anal thing I do is to mark the paint level every time I close the bottle. This is how I use my paints:
When I need certain paint I shake the bottle until the paint is mixed. I open the bottle and clean both the seal inside the lid and the neck of the bottle. Using a pipette I then transfer the paint to my airbrush. Then I tighten the lid and shake the bottle couple of times (to get the paint into the area between the glass lid and the aluminum seal (to hopefully use the paint itself ans an extra seal). I them leave the bottle on my bench until the paint settles down. Then I take a fine-tip Sharpie marker and on the outside of the bottle I mark the paint level and then I write the date like 02-16 (MM-YY). That is not needed but it is just for my curiosity to see how much evaporation happens in how much time. I then store the paint upright in a drawer.
As I said, some of my paints have not been opened for decades and there has been very little evaporation. Having the mark on the bottle lets me know exactly how much thinner I need to add next time I open the bottle.
I know that this seems very anal and time consuming but to me it really isn't. To me it is well worth the effort.
Accu Paint thinner is not just acetone. It also has some other stuff like Polysolve. But acetone is the ingredient which evaporates the fastest. If you need to add a lot of solvent then I would add about a 1/4 worth of the Accu Pint thinner (to replenish the other ingredients) and then the remaining 3/4 of acetone.
Some of my paints are over 25 years old and still perfectly usable. Unlike enamels which harden by chemical reaction (and cannot be thinned by just adding thinner), Accu Paints are lacquers. If they thicken or dry up, adding solvent makes them as good as new.
Vacuum packing doesn't seem like a good idea as it is not the air which makes paint dry - it is evaporation of the solvent. In vacuum the solvent will evaporate even easier than in ambient air.
Also, since Tru Color paints were introduced, there is less of a need to hold onto our precious Accu Paints. Tru Color is basically the same stuff.
