this thread should be in the painting section
but since I already posted to it.... sounds like you did not wash the inside of the body very well or you didn't scuff it if the paint won't stick. If your issue is that the paint is thin when brushed in, well you're right it is. You need patience to brush a body. Just like when spraying, your first coat should be thin/light. Let that dry thouroghly (30 min+). The next coat you should brush in at a different angle to get good coverage, again, let it dry well and do it again until you get the coverage you want.
Hope this helps.