making sure SMC damage is completely dry before repairing..

W

wca_tim

Member
The hood and headlight cover on one of my corvettes (a 2000) fell pray to the trailer hitch on a lifted truck and I'd like to try and repair it. I'm a backyard hack at best, but I've painted a number of vehicles and boats. I've also played around with blending base coat repairs (starting with intercoat clear so the new base pigment will lay down like the old) and burning clear repairs in without leaving a halo using blending solvent, etc... to the point where I have a difficult time finding the repairs I did.

What I'm worried about in this situation is the dreaded bubble under the repair when the metallic navy blue pearl sits in the hot North Carolina. The damage has been exposed to the weather for a while now, so I'm not even sure if I'm able to get it dry enough knowing how water can wick it's way along glass fibers in a composite. Is it going to be good enough to grind out the damage, feather out the edges and then seal it with epoxy (no exposure to anything that's not 100% solids?), is there something else I need to do? or am I on a "fool's errand" even trying?

I did some searches and didn't find this specific issue addressed. As always I greatly appreciate the experience folks on here share.

ps. I painted the hood that's on this car using ppg dbc basecoat over spi epoxy primer and shot several heavy handed coats of spi euro clear over the base. I intend to us the same base and clear for the repair and will scuff and clear all of the panels repaired (maybe even the whole front of the car...
 
If there are moisture concerns, simply heat lamp the area before applying repair materials. I've never found SMC repair to be particularly complex as long as the correct resin is selected, other than that, it is pretty much indistinguishable from a conventional fiberglass repair.
 
crashtech said:
If there are moisture concerns, simply heat lamp the area before applying repair materials. I've never found SMC repair to be particularly complex as long as the correct resin is selected, other than that, it is pretty much indistinguishable from a conventional fiberglass repair.
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Thanks! Some years ago I had a situation once that came from a body shop... there was an area that would make a slightly raised 2 x 4" bubble after a few hours in the hot sun - I was pissed... I gather that it was fairly fresh solvent from a wipe down or thinned sealer, etc... I'm pretty comfortable with fiberglass repairs, I just didn't want to go through the time and energy of doing a good job on the repair only to have to grind it off and start over again.
 
The only critical thing is to select a resin that will adhere, in the world of conventional repairs that I live in, this means an SMC-specific resin that will be activated with cream hardener and not MEKP. That's how you know you have a compatible product. Traditional fiberglass resin won't stick.

I think there are quite a few guys here that might recommend West System epoxies, and they are probably great, but I don't know anything about them.
 
Thank you again - I get the no standard polyester/styrene resins over epoxy thing... I have tubes of lord fusor 100ez, several epoxies (including west systems, MAS and one other along with chopped glass, baloons, etc...) and wasn't positive what I was going to use yet. I suspect any of those would work ok, but not sure what would best match the expansion properties of the hood and headlight cover the best. Leaning toward using the lord product or maybe evercoat vette panel adhesive. Which sounds like the best option?

good thing I don't have to make any money doing this stuff... I'd go broke in a hurry. :)
 
Some time ago SEM posted a good video how to repair damaged SMC. Even if you don't use the specific products it will show you the correct way to repair cracks, holes, etc. You really are only going to need one or two products in addition to the cloth.

 
west system/mas laminating epoxies do work on smc. the thing with smc is that the mold release is actually in the product which is why adhesion is difficult. as always your repair area should always be feathered out with a grinder and 36 grit or a structured tooth bit then it needs to be wiped really well with acetone to remove any residual release that is on the surface.
 
Thanks all very much for your help! I have a plan and wil get to it in the next couple weeks after the differential and drive train are put back together.
 
so I looked at the old hood with the bubble problem and just wow... wtf?!?! (work done by a well respected body shop at the time). A few months after they redid it after the pocket developed under the corner, I caught a boat seat cushion on the center of the hood from a guy pulling a boat the other direction on the highway. I bought a second hand hood and painted it myself (that's what's on the car now and got backed over by a truck / trailer hitch) rather than deal with repairing it.

So the re-done repair area on the corner? I'm guessing this is what happens when incompatible filler is used on SMC and and remaining mold release agent partitions to the surface and interferes with paint adhesion? the first picture is as I found it yesterday leaning against the wall in the attic above my garage / home shop for maybe 5-6 years. The second what it looks like after quickly peeling the paint and loose filler off with a pocket knife.


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I've seen them bubble when guys would used 2K urethane directly on bare SMC. I've seen regular body filler on SMC lift as well.
 
Most modern fillers are rated for SMC. Check the TDS especially if you use off brand or low-end, but that hasn't been an issue for me in many years.
 
crashtech said:
Most modern fillers are rated for SMC. Check the TDS especially if you use off brand or low-end, but that hasn't been an issue for me in many years.
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I didn't do the work on this hood... it was done by a pretty well recommended local body shop that does a lot of business (it was an insurance job...). I took it off and painted a different one after that boat seat missile...
 
Understood, I was just clarifying Chris's comment re SMC, most regular filler these days is actually okay, just have to make sure it's in the approved substrate list in the TDS.
 
crashtech said:
Understood, I was just clarifying Chris's comment re SMC, most regular filler these days is actually okay, just have to make sure it's in the approved substrate list in the TDS.
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Thanks and understood. I have a good understanding of the underlying chemistry, but not the practical experience with SMC repairs... If I wind up using the hood pictured above, I'll grind everything out everything that doesn't look like smc, build it back up up with epoxy and glass, then use an approved filler to finish it up. I'm in no hurry and don't ave to worry about making money or getting a customer car out, so ther's no reason to use anything but the right / best materials. Since I now have two hoods with fairly small areas of damage, I might as well fix and paint them both at the same time for the practice - and because at the rate things are going, it's probably only a matter of time before I'll need another. :oops:

Thanks as always for the benefit of your experience.
 
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