Going to spray my wrx intercooler as it needs a new coat before I install it,but everytime I spray something,primer then a good few top coats,it seems to chip off with just a little knock etc.What do people use for a last coat,satin/gloss clear,any advice appreciated....

Is the primer also chipping off?

Yes on previous painted items.What about enamel paint?

It will be the primer which isn`t taking.....try an epoxy primer, better for aluminium and give it a quick rub down with something like 320 grade paper.