Watery white and yellow gundam marker

so…I have two different yellow and white gundam markers from the seed set, and the basic set…and the white and yellow paint has ALWAYS come out watery, and I do mean watery, like its not mixed the pigment in with the suspension medium properly…am I doing something wrong here?

is there something I can do to prepare the plastic surface for better paint adhesion eg roughing it up a bit?