Gen'l questions - Paint, care/storage, thinning, etc.

I can tell you that I have had containers of Tamiya for years with no elaborate procedures,whatever paint you use,you don't want to put thinned paint back in the jar,it will shorten the life of the jar.Also you don't want to subject your acrylic paint to freezing temps.My enamels and lacquers have done well also,I've heard don't shake the jars,and keep the threads and inside of the caps clean.

For thinners,I usually stick with the companies thinner to avoid difficulties.

Also get out the model paint you need with a pipette and get the container closed back up right away. Tamiya in particular because of the wide mouth jar and alcohol based acrylic paint. You get painting away with the bottle open and forget it you can lose some of the base. I haven't gotten a skim on paint, be that 1/1 automotive paints or model paints since back in the days of alkyd resin paints like Dupont Dulux, and as mentioned in particular if thinned paint is returned to the container. Even Testors enamels do fairly well if you keep the lids and threads clean ( also mentioned already).

I use a Badger battery powered mixer to stir settled pigments back into the mix if I see that going on. I learned my lesson on that with Model Master acrylic, you can shake your brain out and never get it all mixed back in, where 30 seconds with the mixer and it's done. Stynylrez primer too is like that.

Ditto on not letting acrylics freeze. It hasn't happened to me but I've read where it has and ruined the paint all together. I have 10 yo craft paint that's still good but who knows what happened to one every now and then that has gone belly up. Most of those were my wife's before she passed on so who knows the total history, she bought many but were given some too and those could very well have froze before she even took possession.

Some folks complain about Vallejo Model Air paint, I've never had an issue but I do thin a bit extra with my own blend of thinner. Even so they usually spray right out of the bottle, I thin mostly to get some retarder in there and because I use a fine needle a lot with those paints ( .25).

I think Rustloeum POR is probably a resin or alkyd paint, last man standing so to speak but I could be completely wrong about that. The only quart sized cans of finishes I use anymore is Minwax clear lacquer so I should just be quiet. I haven't sprayed Rustoleum in maybe 40 years, it was enamel back then (of sorts). I cut it with 3812, the same stuff I thinned Dulux with. Sprayed great.

I'm still painting with 34 year old Tamiya paints in the large bottles.

I've got a few mason jars with decanted Model Master rattle can paint in them that are still in great condition after years of storage.  For no apparent reason though, the one that had Camouflage Grey in it turned into a solid block.  It might be possible that some paints don't do as well in storage due to their composition.

Other than that, I don't really have anything to add to the great advice already given.  Paints definitely do better when they're stirred and the caps and threads are kept clean.  Like OMG said, take a little bit of paint out of the bottle and close the bottle.  Usually when I brush paint, I use my Badger stirrer and just what's left on the stirrer is usually more than enough to drop into an aluminum paint pallette and brush paint with.  So you can transfer it that way as well.  Just try to avoid shaking the paint and pouring the paint...both things muck up the threads and caps.