Professionals look away now, lol. I'm in the middle of a quick cheap touch up on a 20 year old camper van bonnet. Using Holts white primer in prep for a holts white topcoat. I've used a sander to feather back the old paint in one area. But the now the paint reacts and blisters with the some parts of the old white top coat where I feathered back. Any handy solution for fixing this. The more I sand it away, the worse it gets.thanks