Does this sounds at all familiar..???!?
I tend to use the backside of bad picture prints to make colour mixes, but also to put drops of PVA glue or superglue (the superglue is then transfered to the model with a cocktail stick).
I normaly use different pics for glues and for paints... but sometimes I don't.
Can you already see how this is going to end..? [:0]
Nothing more frustrating than refilling your paint brush with superglue, I can assure you..! Aaaaaargh!