They are ok mate I have had some for YEARS (around 8 or 9 I think), the pigment will settle but I've not had any significant evaporation I don't think. I will have to check again. If you do get some evaporation it will most probably be the solvents, just add some 2k thinners/reducer and you will be ok. Are you ordering from DeltaArts? If so he is good.
Lee