Auto body repair & detailing: 1978 Caprice collison and repaint., primer surfacer, hairline cracks


Question
My Caprice Coupe was an original car prior to a 17 year old girl running into it. I have replaced the damaged parts and I am prepping the car for an entire repaint. I sanded the car with a long board mostly to factory primer and a few areas of bare steel in the highs and body color in some lows. I skimmed the few minor dents with thin ice, and applied duplicolor professional primer surfacer to most of it, which I will sand and coat with a sealer. I am afraid of the roof and the hood, however, because the original paint has what looks like tiny hairline cracks all the over it. I've tried sanding it with a board sander and 36 grit paper, but go through to steel in a few spots but barely cut into the paint in others, which leaves some of the cracks. The hood didn't look like it was that wavy before, and I am not sure if I can lay the primer surfacer on it with some cracks in spots, sand it and seal it, or if I need to makes sure to remove all of the cracks, and skim it to fill in the lows. If I go that route, I am afraid that It will turn out wavy because it is so large and flat (roof too). Suggestions?

Answer
The cracking is what's called checking, and the only repair for checking is to strip the affected panels down to bare steel, fill any small dents while it's still bare, etch the steel with metal etching primer, then seal with an epoxy primer, then primer surfacer is applied. Anywhere you apply paint over a checked surface will swell the checking, and it will show through the topcoat. Sometimes, it will REALLY go wild, and the topcoat will horribly react to the old, checked paint and the new finish will dry and look worse than it did before it was painted! If a car is just peeling, saving the factory base primer is desirable. If it's checking, it all must be stripped. Here is a very important tip- your car has factory lacquer paint on it- if you use a 2 part catalyzed primer of any sort on it, IT MUST BE FULL PANEL PRIMED!! Spot priming will result in the new finish attacking the primer edge around the spot primered areas. Lacquer and catalyzed products don't mix well, lacquer is a thermal flow product, catalyzed products are thermal set. A catalyzed product is easily identified, because they have a hardener or activator they must be mixed with. I'm here if you need any other pointers. Good luck! I love seeing these big old boats painted, and not run in a demolition derby. Bill