I know very little about paint, but it sounds like its simply seperating.The pigment is probably stuck to the bottom like gum on an old shoe.
Hot water, soak the can for while.Then shake very vigorously.
I had some old Con Lux industrail paint(the good stuff if you know what I mean, this is the same type of stuff they painted the Golden gate Bridge with) gave it to a friend in pa that painted a steel oudoor structure, he gave me a case of Heineken with a big smile on his face, so I guess he was happy with it. I also pained a fire escape in NY with it, worked fine.
They still sell the old Rustoleum stuff on ebay,so it must be still good after 20 years ,long as its totally sealed.
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