How do you know when paint "goes bad"

[WelshZeCorgi]

I mean, does paint even go bad?

 

I've noticed that some Tamiya paints I have come out of the airbrush as smooth as water while others seem to spray out a course, large bead pattern or cause the brush to spit and sputter, clogs the nozzle easily. Some of these paints are nearing 4 years old and while most of them are well maintained, the ones I use the most have paint caked up around the lip of the bottle, causing the caps to get glued shut. I'm guessing that those bottles don't have an airtight seal and would cause the paint in the bottle to grow thick over time. 

[dnl42]

Some paints age well, others don't.  I can use Mr Color even when it's so thick it's a gel--just needs more thinner to get to airbrushing consistency. When I used Model Master enamels, they would definitely get too old. I don't know anything about Tamiya's aging.

 

As for the jar and lid, I always wipe down the jar's thread before I close it up again to prevent air ingress or a painted-shut bottle. I also don't shake the jar to mix paint, I use a mixing stick to be thorough.

 

My current mixing equipment consists of a Tamiya paint stirrer and a small stainless steel paint mixing cup from the local art supply store. A clean-up with hardware store lacquer thinner, same stuff I use for all clean-up, including the jar threads, and I'm ready for the next color or session.

[modelingbob]

Yes, paint goes "bad" over time. The time varies widely. I have some old Humbrol enamels that are over 20 years old and still work great. OTOH, I have some Testors MM that I can get barely a month or two out of. You can usually tell if the paints gone "bad" pretty easily. Not thinning smoothly, big clumps of solidified paint, pigments separating out, etc are signs that your paint may be well past its prime.