Washing Meng kits before paint

I never wash before painting and have recently paid the price twice. I had no problems with Meng's D9R dozer kit but the French AUF1 was actually greasy feeling in some places and paint didn't take well at all. A simple washing with spray cleaner and then soaking in dish detergent did the trick. Now I'm doing their amazing M2A3 Bradley and once again find the need to wash (not that I did until too late). As an aside, if you love full interiors with working doors and hatches you've got to try all three kits. The detail is terrific. Buy them all and when you do maybe someone can tell me how to make the dozer tracks because I can't. They changed the track building system on each kit so the AUF is easier and the Bradley a snap - literally.