I have taken my cars to the same body shop for the last several years and notice that the paint is developing sanding scratches in the paint after several months after leaving the shop. The process the auto body shop uses is as follows 1) strip car down to bare metal 2) seal with epoxy primer 3) do body work 4) Prime with 2K primer and guide coat 5) Block car 6) reprime with 2K and guide coat again 7) block sand and prime again with high build 2K and guide coat 8) final blocking 9) spray car with sealer 10) apply base coat 11) apply clear coat. The Paint looks great but after several months I notice sanding scratches that were not in the paint before. I believe that the paint is shrinking. I have heard several people say do not use 2K primers as the shrink, Can anyone help me with this problem?

Dan

it isnt the body filler (bondo) shrinking is it? if thats the case, by allowing the car to sit and let the bondo settle in, it will not shrink under the paint giving the scratches and possibly formation of dents in the body work.

I feel confident that the bondo (filler) is not the problem. The car is laser straight no ripples or dents. I feel that the problem lies in the primer that is a high build sandable type. Does anyone have problems with these sandable high build primers shrinking? The sand scratches don't appear till after a few months after paint is applied and wet sanded then buffed.


Dan

is it sanding scratches all over the whole car or just in spots where body filler was used....most of those primers are are used with a hardner and i haven't seen any shrinking problems using the 2k primers. if it's all over the whole car i have seen some cars show 1200 to 1500 sanding scratches after several months if the car was not buffed out correctly. if the car is hit with some finer paper and re-buffed they will go away.

Part A.) The problem is not the primer shrinking, not entirely. Because even a 2K will slightly, but not like the lacquer primers we used years ago (although I have friends who still use it without any problems). It also must be catylized properly. The coarse sanding scratches must be removed with a finer grit (180, 220 to be safest) if the prime to paint process is done quickly. If one primes over 80 or 100 grit scratches as 2K is perfectly capable of doing, and does not allow sufficient full cure time, subsequent pre-mature coats can develop scratch appearence. As an analogy: Pull the plug out of the bottom of a liquid filled container and the effect on the surface is delayed. Such may be the case with your scratches appearing later. In most cases, the shrinkage observed can be buffed out if they are not severe. Smooting the clear to change refraction/reflection. When you pick up a car in the future make sure you see no scratches in the basecoat. They will not buff out. What you are experiencing is not uncommon. My cars usually sit for some time so this problem is not encountered. If time is pressing the above procedures should be followed.
Part B.) Sometimes final wetsanding prior to buffing is not done with a 2000 or finer grit to ENTIRELY remove the coarser cutting paper scratches (ever buff stainless?). If the clear is not fully cured it will shrink as well. Customers want their cars back so fast after they are painted they generally don't want to wait while it sits at the shop to cure. Baking booths usually eliminate this wait time. You may have to bring it back for rebuffing. Patience disregarded results in more work.

Yoy may have touched on something here I don't believe the guy used 2000 or finer for the final buff of the clear. The paint is not baked in a booth. So what would you say would be sufficient time to wait before buffing and sanding the clear after it has been sprayed?


Dan

I waited 10 months... Just was perfect timing and they hit it with 3000 grid and cut then rebuffed... Looks great

More than likely you are seeing the final grit sand paper used right before sealer. What color is it and what grit paper was used? How long was it in prmer before color was shot?

Are the scratches cross hatched or in line? that may tell you weather it is in the primer or the color sanding.

2K primer most certainly can shrink, and shrink bad! IF the primer is applied too heavy, not enough flash time between coats, too cool of temp or a number of other factors. Piling the primer on full of solvents will slow down the cure, even stop it when you introduce other issues like cool temps.

Some poor quality 2k primers aren't even solvent insoluable so even if applied correctly if it is piled on too thick and you are asking it to do too much like fill 80 grit scratches or something like that, you are likely to have it shrink or "swell" which can look pretty similar.

Brian

2K primer most certainly can shrink, and shrink bad! IF the primer is applied too heavy, not enough flash time between coats, too cool of temp or a number of other factors. Piling the primer on full of solvents will slow down the cure, even stop it when you introduce other issues like cool temps.

Some poor quality 2k primers aren't even solvent insoluable so even if applied correctly if it is piled on too thick and you are asking it to do too much like fill 80 grit scratches or something like that, you are likely to have it shrink or "swell" which can look pretty similar.

Brian
All very accurate points.And also,not all 2 pack paint systems are equal these days either.The shop should be wiling to sort this out for you with no drama.

Color is medium blue, final block sand was 320 I would say car sat for 1 week before sealer went on.

Dan

Scratches are in line not cross hatched. I see what your saying then this may point to a clear coat issue with the final sanding.

Dan

320 is too coarse for even solids IMO. 400 should be the minimum, 600 is better, 800 or finer if doing metallics. Do you know what grits were used to color sand?

320 is too coarse for even solids IMO. 400 should be the minimum, 600 is better, 800 or finer if doing metallics. Do you know what grits were used to color sand?

If the shop did in fact use 320 its not a good idea. I never use anything coarser than 600 prior to paint. More comfortable w/ 800. We cannot see your problem. Also, as I said before, not color sanding the preceeding coarser grits well enough with a finer paper will leave those coarser sanding scratches showing after it is buffed. They have to be completely sanded out prior to buffing. This may be what you are seeing. Naturally there may be touch up but this should be resolved before it leaves the shop. The shop should show you this can be done while you wait and then schedule a once over to rebuff.