Question about painting

How come when ever I apply a basecoat to a model that I have primed, the coat is always bumpy? My paints are slightly watered down although no flow improver.

I use a pallet of warm water with a drop a washing up liquid, I add it to my base coat until it's like milk :-)

How old is the paint you are using?

What colours are using for the base, I may have some old paints that I'm not using that may be in better condition if that is the case of the lumpy paint.

Some of my paints aren't that old, especially the ones that I was using. I get the feeling that it was because I didn't wash the model before painting, he has been sat on my shelf for about two weeks.