preserving acrylic/enamel paints over time ...

I agree with Wilbur.  I find my thinned paints do age faster than the unused bottles.  It usually lasts long enough to finish the kit I work on, but I have tried to use a bottle of thinned stuff in a bottle, from a previous project that was a couple months old and it had polymerized- soft but unthinable and unusable.  I have noticed that on any number of old bottles previously thinned.

Wilbur Wright

Don't ever introduce thinner into the original paint bottle it will ruin the paint faster.  I have never had a problem with long term storage of Tamiya acrylic paint. Model master enamels will go bad much faster.  Gloss even faster.

Anything thinned for an airbrush in an airbrush bottle can't be stored that long.  Never pour that back in the original paint bottle, it will end up ruining the original paint.

 

General tip is to wipe the rim of the bottle clean.  Right down to the glass. Then use a Q-tip to wipe around the inside rim of the lid wiping up any paint there.  Then hand close very tight.  This insures a tight clean seal.

 

The first part...I can agree and disagree. I use nothing but MM enamels...and I do dump thinned(for airbrushing) paint right back into the bottle. For the most part, I have no problems with the longetivity of the paint. That said, there are some colors that I will never see the bottom of the bottle...blue is a great example...whether or not it has had thinner introduced. I have some colors that have had thinner added up to ten years ago, that I can still use without issue, I've had others turn sour in a matter of a couple months. It's a bit hit-r-miss.

The last part...100% agree...keep it clean! In addition...I have found that my paint lasts a lot longer if I stir, rather than shake. I make a "T" out of scrap sprue and use a Dremel (set on LOW!!!!) to stir it up. Huge difference since I started doing that!

Don't ever introduce thinner into the original paint bottle it will ruin the paint faster.  I have never had a problem with long term storage of Tamiya acrylic paint. Model master enamels will go bad much faster.  Gloss even faster.

Anything thinned for an airbrush in an airbrush bottle can't be stored that long.  Never pour that back in the original paint bottle, it will end up ruining the original paint.

General tip is to wipe the rim of the bottle clean.  Right down to the glass. Then use a Q-tip to wipe around the inside rim of the lid wiping up any paint there.  Then hand close very tight.  This insures a tight clean seal.