Looking at a set of half doors. Seller wants $550 for set with newer uppers but no mirrors. Issue is rust in the pinch welds at bottom. What causes this? Water instruction from above then can't drain out bottom? Is this a serious issue or can I just sand down and repaint doors. Aside from this they appear to be in pretty good condition. Should I keep looking? Not like there is a plethora of half doors for sale.