WHAT AM I LOOKING AT?

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PostPosted: Sat Jul 02, 2022 2:47 pm
So I finally got around to my hummer h2 SMC hood. I have checking in the paint and am going to repaint the hood. I did an exploratory sand to come up with a process to help sand away the checking and not screw up the SMC.

I attached a picture and I am not sure what I am looking at. I see the clear, then base and some type of primers (white and gray). Using magnification, I can see the checking pattern all the way down in to the primers. When I got to the SMC, I think that is what it is, the checking is finally gone. I thought I could just sand of the base coat.

Ted
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PostPosted: Sat Jul 02, 2022 6:46 pm
what i would try first is to sand down to the gelcoat of the SMC. then clean it good and wipe it with some w&g remover or a reducer of some kind and look closely for any checking. i've done a lot of SMC stuff, canopy's, removable tops, corvettes and so on and i don't recall ever seeing the actual SMC have a checking problem. BUT?
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PostPosted: Sat Jul 02, 2022 11:32 pm
So what you have run into makes sense..... Sheet molded and bulk molded heat compression molding cannot use any type of regular gel coat in the molding process. It is simply a resin compound and the fiber materials mixed together by injection in most cases. What appears to be the final gel coat right before nicking into the resin/fiber is just a heavy duty primer/surfacer which is the first coat of anything shot on the fresh parts when cooled enough. I have seen it craze/crack before......
Years ago I toured the Corvette plant in Bowling Green. On the tour I got a glimpse of an area where bright white raw SMC stuff was going into an area and then you saw those same parts later with a dull gray surfacer on them.....
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PostPosted: Sun Jul 03, 2022 7:52 pm
badsix wrote:i've done a lot of SMC stuff, canopy's, removable tops, corvettes and so on and i don't recall ever seeing the actual SMC have a checking problem. BUT?
Jay D.



Including this one!
Looking all around perimeter of pic out side of sanded area I don't see anything?
No Checking no cracking no chicken feet.

I do see Way too aggressive sanding.

But for arguments sake say he does Have checking,
Usually caused by excessive film build.
Only way to cure it is to strip it and repaint.

I would be real Leary of chemical strippers on SMC
Sanding it all off is gonna be a Chore
Beware of sanding into SMC and damaging it.
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PostPosted: Mon Jul 04, 2022 7:48 am
Yep, agree with Dennis on that one.... SMC + Chemical Stripping = Surface Meltdown.....
Razoring, Scraping, and Sanding will be your new hobbies.....
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PostPosted: Mon Jul 04, 2022 3:11 pm
Thanks for the answers. Once I got through the clear I used only 220 grit and went slowly because I did not want to damage the skin coating on themSMC. Imam really worried about the molding release agents that are used on newer SMC panels. They are built right into the compounding material and can be released if one damages the skin.

Anyway, here is what the problem is. They really showed up when Buffed the vehicle and the little checks filled with polish. The checking never occurred in the SMC. However, it was in all four layers. Thanks for the explanation of the layers. Very little material was laced down and once you get through the clear it almost rubs off. That would be my preference I think? Using something to wipe it off after I get down to paint or beyond so I don’t damage the outside coating on the SMC. Lacquer thinner maybe?
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PostPosted: Mon Jul 04, 2022 3:13 pm
Another view. Only on the top of the hood of the truck.
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PostPosted: Mon Jul 04, 2022 10:31 pm
Good intentions are stuffed into this phrase.... " it almost rubs off." Lacquer thinner would be one of the worst things you could do to this. It could be a contamination nighmare after that. I think sanding is what is called for here. There are some new media types that would be acceptable for this.... plastic sponge blasting and deformable plastic pellet blasting. I was talking to a powder coating guy about different types of media available to him. These came up.....
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PostPosted: Tue Jul 12, 2022 9:09 pm
Dontheo55 wrote:Another view. Only on the top of the hood of the truck.


I don't know what you got their BUT it is NOT Checking.

Pics not very good cant zoom in on them please try up loading to this web sight.

By the looks of the heavy sand scratch that was Painted over you probably want to respray this hood over again anyway. I would.
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PostPosted: Wed Jul 13, 2022 10:21 am
Dontheo55 wrote:Another view. Only on the top of the hood of the truck.


I've seen this on fiberglass when solvents were still trapped before spraying.
I don't know about SMC, but if it's like fiberglass you never ever get it
wet with any solvent, especially lacquer thinner and even "wax & grease removers"
Solvents will get trapped for a long time and come back later to bite you
when you're dealing with plastics.
JC.

(It's not custom painting-it's custom sanding)

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