I have a couple of tins of humbrol metallic paints (copper, brass). I have used these once (some years ago). I don't know if they're defective or what, they behaved strangely. When I stirred the paints and started painting, the paint was fine, but after about 10 seconds, I noticed the metallic flakes were floating to the bottom. I had to stir the paint everytime I needed more. After about half a minute, I was brushing on just the fluid with a few metallic flakes in it. Also, these paints were very sensitive about thinner. They lifted off very easily when I tried applying a black wash.
Are these two paints defective, or what? A local hobby shop keeper said that paints may behave like that if exposed to cold during storage or transit. Other humbrol metallics work okay, they don't require stirring every 30 seconds and withstand carefull washing with thinner without immediately lifting off. Has anyone encountered these kind of problems?
Should I just buy another two paints and see if those work okay?

I've encountered similar problems with SnJ metallics, after a couple of years they age and don't maintain the metal in suspension as well. No cure I now of, replacement is the only answer I've found.

PS - Strangely I have some Testor Copper that is 20 years old and is still fine ?

I guess I'll just have to buy some new paints and hope they work. Citadell's acrylic metallics work fine and look good but sometimes I like to use enamels as they can be buffed.
Cheers