Is old paint any good?

I have paint on the shelf that is well over twenty years old and still perfectly good. If it disperses properly, apply some to a piece of CLEAN styrene scrap and allow it to cure overnight.  If the adhesion is good the paint's fine. I would, however, strain it before using, as there are bound to be some tougher clumps of pigment that may never disperse.

Metallic paints are another can of worm. They don't have as good a shelf life as other paints due to the very unusual nature of their pigments. For those, I'd definitely try a fairly large test sample before using them on a kit. 

As long as all the caps are screwed on tight, you should be fine. I grabbed a bottle of Testors flat yellow the other day and it was nothing but a bottle of goo since the cap was loose... I'm a notorious pack-rat so I saved it anyway...

Maybe I can chuck in the drawer with all those used batteries

Jeff

I've got jars/tins of Testors, Humbrol & Pactra paints that date back to the late 60s, early 70s that are perfectly usable.  Probably the biggest thing that I do to ensure they remain usuable is carefully clean the top of the jar and the inside of the lid after I've used them.  I simply wipe down those areas w/a rag that has been dampened with thinner and then store them in an upright position.

Salvaging paint that has started to go bad is a 50-50 proposition.  If a jar/tin/can of paint sits for a period of time, the solids will precipitate out of the mixture and settle to the bottom (and thats true of any paint).  After a period of time, if the paint is not mixed, a layer of hardened translucent material will form between the solids and the liquids.  If the layer is thin when you discover it, you usually can salvage it - simply add a little thinner (lacquer thinner seems to work best), break up the surface as best you can with your stiring stick/rod, and then stir well.  If you have lumps, you'e going to have to strain the paint - some material from a set of panty hose works very well as a strainer (ask your wife or mom for the next set she's going to throw out or if push comes to shove, visit the Dollar General and buy a pair).  Simply (right!) pour the paint through the material into a new bottle.  Note: the surface will often continue to thicken untill it gets to be about 3/32 to 1/8 inch thick.  My rule of thumb is if the layer is 1/32 to 1/16 thick, I'll try to salvage it - any thicker and I simply throw the bottle away.

Suggestion:  I've noticed that I have a few older paint colors that are no longer available that I tend to use more than others (Pactras Hot Rod Primer & Humbrols Doped Linen come to mind).  I have started to make provisions so I will always have a source of those colors when the origionals run out - I paint a 1" swath of the origional color on a 3x5 file card, marking the card with the paint manufacturer, the color and the stock number and store it.  When the origional runs out, I'll mix a new batch out of current available paint and then add a 1" square of that to the card, putting the mixture on the back. 

Sorry to get so long winded