Hi Andy! I've had problems with matt black, worse with satin black. Also, REAL major problems with at least 3 different greens, and matt white and satin white. At best, after at least TEN MINUTES of strirring, I've been able to just about use them for brushing, but not for airbrushing. ALL of these paints have 'gone off' within 3 to 4 weeks of being opened. I have some Humbrol paints that are at least 20 years old, which have been opened and used, but are still as good as new. BTW, the latter are some of the colours only used now and then, hence their age!
Terry.