Paint chips in relatively new paint job

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PostPosted: Wed Aug 31, 2022 5:57 pm
i hate to be the one with bad news. but filling chips is going to cause a problem, the filler is going to shrink in a month or so and with that black finish you're going to probably see it. it might depend on how deep the chips are when you fill them. i would sand down past the chips and feather out (a lot) spot prime with hi-build and block. at this point i would just spot prime your repairs with the hi-build then base. with the black im not sure i would use a sealer you're getting a lot of coats of material on there and the black will cover good. lets see what others have to say?
Jay D.
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PostPosted: Thu Sep 01, 2022 6:36 am
gprs wrote:Now, the paint being too thick is not something I considered. Do you mean that one (or more) of the coats was put on too thick or that there were too many coats?


Either, or both. When the total paint thickness exceeds 200μm the propensity for chipping increases exponentially. It's one of the reasons companies like Honda take great care to keep their paint thickness down - much less likelihood of chipping, certainly large chips like you have. However, thin paint can have other downsides, like increased likelihood of clearcoat failing and Honda, for one, address this by using much higher quality products that will give the same or better paint life as competitors.

How thick is 200μm? Good quality factory paint usually runs about 130μm (anti-corrosion, primer, basecoat and clearcoat). Two full, wet coats of clear can easily run to 60μm, so it doesn't take very much extra paint to start getting into the danger zone, especially for inexperienced painters who tend to use way more than is necessary. A layer of high build can easily take you over.

That's not to say that over 200μm is "too" much, that only comes into effect over 300μm, but propensity to chip certainly increases.

gprs wrote:The rest of the story is that I mistakenly only used plastic masking the first time. I subsequently learned that paper should be used near the area that is being sprayed otherwise excess paint on the plastic turns into flakes and blows all over your newly laid paint


You just used the wrong plastic. When plastic first came out I tried it and it was a disaster. Job after job ruined because of the flaking. We went to a paper border and that fixed the problem but we lost many manhours with the extra work to mask up. Eventually better quality plastic came on to the market and it's now all I use. I can prime, sand base and clear with a single masking and have no flaking problem at all. Of course, I wouldn't leave it there for a couple of days but if it's a wet-on-wet application it's great stuff. $40 gets me a roll that's 120m long and 4.8m wide. Pretty cheap masking.

gprs wrote:clearly, something went wrong


Yep.

I think that P400 is too coarse to prep for basecoat but I don't think that's the issue. Maybe a contamination? Maybe your second base application soaked through and wet the bond to the primer. Possible, but who knows and you'll be unlikely to narrow it down if it's a one-off.

gprs wrote:the recommendation is to put a coat of epoxy primer down first, then urethane, base, and clear.


Only If you're painting bare metal and only if the surface isn't good enough and warrants the use of the urethane primer to level out. Otherwise, epoxy, base clear is just fine.

badsix wrote:i would sand down past the chips and feather out (a lot) spot prime with hi-build and block. at this point i would just spot prime your repairs with the hi-build then base. with the black im not sure i would use a sealer you're getting a lot of coats of material on there and the black will cover good. lets see what others have to say?


I agree with Jay if you were repairing a factory finished panel. But there are doubts about how well your base is sticking to the primer, evidenced by the chips. I'd suggest that the only way you're going to fix this properly will be to strip the bonnet back to bare metal and start again. Painting over a questionable substrate will only make the problem worse.
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PostPosted: Fri Sep 02, 2022 7:35 am
Wow. There's so much to digest here. Thanks very much for the detail.

So, don't fill chips, feather them: makes sense. That nugget will save me a huge headache.

Honestly, paint thickness as described never crossed my mind but I can see how it would make a difference. With the failed first attempt covered by a second attempt, I'm sure this is a good part of my problem.

I've read and watched quite a bit trying to learn how to do this well and it seems there are some that are happy with P400 and others that take it to P600. Honestly, one extra round of sanding doesn't bother me so I may just try P600 this time.

I didn't _plan_ to go back to bare metal. The surface is in good shape aside from a few small areas that had some surface rust or tiny dents, I only went down to the original paint.....oh....hold on a tick. I'm not the first to paint this hood. The primer and failure to bind is all me but this hood's been painted before so there's likely even more paint than I initially thought making that 300μm almost a certainty.

So, new plan.

Down to metal I go (which honestly terrifies me a bit), re-repair any bondo'd areas, black epoxy primer, base, clear.

That'll be fun.

Thanks again for the detail and patience. It's very much appreciated.
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