preventing acrylic/enamel paints from thickening/solidifying over time in storage ...

No expert, but here's a suggestion for storing paint of any kind. Best to avoid getting paint on the threads of the cap or the bottle, that allows air to hasten the drying process. That occurs when dried paint disrupts the natural seal of the cap/bottle threaded surfaces.

I don't pour paint over the sides of the bottle and I don't shake the bottles prior to use. Shaking the bottles simply gets paint up and onto the cap interior, which in turn has a negative effect on the cap seal. Best to stir the paint carefully, keeping it all in the bottle and below the cap. Stirring is really the only way to ensure getting all of the settled ingredients from the bottle bottom, throroughly mixed into the entire contents.

I transfer paint from the bottle into whatever I will be painting from by use of a pipette or straw, keeping the entire upper bottle surfaces clean. If you must pour from the bottle, then at least give a good cleaning of the bottle top and threads, plus the cap as well.

Most members that do keep the bottles clean, relate that paint has been stored for several years and still perfectly useable. It works for me as well.

Patrick

I work mostly with Testors enamels, so don't know much about storage of acrylics.  With the Testors enamels, I find unopened bottles do last a number of years.  Once I open them, they do start to thicken, maybe last a year or two.  Once mixed with the thinner I use (mineral spirits or turpentine) they degrade rapidly, and congeal in such a way that they cannot be thinned or used- they polymerize into a sticky mess.  Last a couple of months.

I have realized that the cost of paints is not a major expense compared to price of kits or aftermarket accessories, so have been buying fresh paints for major colors of a model.

I use primarily acrylics anymore and have follow a similar method of use by Patrick. I went on eBay and bought a large bag of pipettes to remove paint from the bottle to a small mixing cup that I can either pour directly into the airbrush or use as a paint pallet if hand painting. I bought the cups on eBay as well, they are the cheap plastic medicine type cups. Since following the rule of wiping the rim and threads of the paint pots after use, I've found that my acrylics last MUCH longer than before. I also use a lot of the Ammo of Mig and AK products, they come in small "eye-dropper" style bottles and the same rule applies, wipe away any excess paint and they will last a lot longer. The only difference in those products, than say, MM/Testors or Tamiya and Humbrol paint is that they are recommended to be shaken as they contain a small bearing in the bottle to facilitate mixing and this does cause a small amount of paint to come into the tip of the cap.

Like Don, when using enamels, if the bottle is taken care of they will last a long time if they havent been opened. Once you get paint on the cap and the paper seal inside the bottle sticks when you open it, it isnt going to last much longer than a couple of months at most, again, cleaning your caps and threads will help.

I never pour paint that I've added thinner to back into the original container. IMO, you are only reducing the original products life span and will get a gooey mess if it sits for too long. If I've mixed paint and I have left-overs and I know I'm going to use it again fairly soon, I'll find an empty bottle and store it in that, if I'm likely not going to use it again, I'll just trash it.

In storage, pigments and other solids tend to settle to the bottom over time. Some brands of paint are more susceptible to this than others. The longer they settle, the harder it is to reverse the process. Often, a vigorous shaking of the paint will restore it to a usable state. And yes, it is ok to shake your acrylic paints (unless the directions on the bottle tell you not to). If the lid is on tight, drying out shouldn't be a problem. I have opened bottles of Tamiya paint that are 9 years old and still usable after a good shaking.

If I am struggling with a particular bottle of paint and shaking isn't getting it done, I will use a Badger electric stirrer to mix the paint. This takes much less effort on my part, but it wastes a little paint every time I use it. Sometimes it needs to be stirred for a full five minutes to be effective. Also, the stirrer is just a little bit too big to fit into Vallejo bottles, but that can be quickly fixed by carefully grinding the mixing blade just a bit.

Preventing the settling of solids will take some work. You can store dropper bottles on their side and periodically change their orientation. This takes effort to do, of course. What I do with my Vallejo bottles is store them is a little fishing tackle box. When I am not painting for a day or two (or longer), I will turn the tackle box to one side or the other. I don't find it necessary to do with with Tamiya paints.

The solids will still settle when I do this, but with the periodic change in orientation, it keeps the solids from sitting in one place for months on end. This makes it easier to shake the paint to a good mixture when I need it.

I have noticed that the new Testors paints I have tend to go bad sooner than the older ones. I have some Testors enamels that are 25+ years old and are still good while ones that are less than a year old have gone bad.

If you store your paints for a long time it's best to rebottle them and one time a week or two shake them for about a min each.Testors mixing bottles works best for storage.It comes w/ round marking stickers for each bottles.They come in a five pack.

The trick is to keep air from getting through the threads on the bottle.  Keep the threads CLEAN !    I usually get 5 or even 10 years use from a bottle of paint.   I do however have a bottle of Pactra  light blue metallic  enamel from 1977 ..... with a price of 35 cents printed on the cap..... that I do still use. Its almost gone so I guess  next year I will have to find a suitable replacement.