Shrinkage

[iroc343]

I am a hobbyist who, from time to time, will tackle my own body repair. The problem I'm encountering is paint shrinkage. It usually shows up a couple of weeks after the finish is applied as sand scratches. I always use quality products, body filler, primer, and top coats, and I'm not using an excessive amount of filler. I am wondering if the application of a sealer would help. It's frustrating to go through all that work and produce a professional finish only to have sand scratches appear weeks later.

[LearnAutoBody]

Be sure to allow each layer of filler or primer to fully flash. Putting primer on to thick or rushing the flash times between coats can cause sand scratch swelling. Do you use a catalyzed primer? The old lacquer primer was real bad about doing this. Be sure to use 2 part primer surfacer.

I think the problem is applying the products too thick and or not waiting long enough between coats. I usually let primer dry longer than the minimum it says. Overnight if possible.

Flash and dry times are key. Do you think you may rushing flash times or not allowing to dry long enough before sanding?

[Mark Lapes]

I have a question on this subject.


Can shrinkage cause cracking in the paint??????????

[LearnAutoBody]

Mark, I have never seen that. However, I'm not saying it could cause cracking. I am not real sure about that question. I guess if 3,4,5 coats were hosed on back to back it may cause that to happen. Not sure! To be safe wait the recommended flash times. Remember these recommendations are usually set for 70's degree weather. If it is really cold in the shop, you are going to have to extend the flash times.

[iroc343]

I did use a catalized primer. Sounds as if I didn't let the primer dry long enough before applying the top coats. It was a weekend project on a daily driver, so time was limited. Thanks for the info.

[chaswe75]

The question is what products are you using? Scratching swelling in body work can be caused by:
-cheap primer that shrinks too much and not applying enough primer film build
You should be putting 3-5 coats of a quality urethane or acrylic primer. Also most people do not realize that the final scratch in body work should be softened up before priming. 180 or some people do 220. I DA all hard block scratches that are on the edge of the body work with 180 then 320 feather-edge. You can prime over some hard scratches with a good primer and possibly get away with it. I have seen body work scratch swell months later by doing this though, especially with cheap primer.

[LearnAutoBody]

The old lacquer primers are bad about shrinking. You can still buy that kind of primer too. Be sure the primer that you use has a catalyst.