SMC or FRP??? c2 inner fender issues..

[Jim V.]

Is this replacement inner fender on my 65SMC or FRP...? Has noticable fiber strands on the motor side but heavy resin especially on wheel side. How is FRP differentiated from SMC?

If it is SMC, will Evercoat Rage still be the filler of choice on the pock marked motor side of the panel?

Top picture is a pic taken from the motor side showing the pock marks while while the lower is obviously the wheel side after I stripped the undercoat and paint. The wheel side is also quite glossy with a lot of resin and little sign of fiberglass fibers. When sanded the motor side shows much more fiberglass fiber.

I have scuffed up the wheel side in preparation of refinising with undercoat (3M) and blackout (Krylon 1613).

Thanks....

[Chuck S.]

Re: SMC or FRP??? c2 inner fender issues..

In your case, the key difference is that SMC (Sheet Molded Compound) is SMOOTH on both sides. The reason for this is the composite material is first rolled into a constant thickness sheet (or possibly onto rolls for bulk manufacture). The part is formed later by placing the already formed sheet of material into a heated molding press. There are other differences like color and less visible reinforcing fiber...SMC also tends to more tough and flexible, while FRP is fairly rigid up to it's fracture point.

As I said before, I'm pretty sure your inner fender is a hand-laid panel of polyester FRP. You would expect the mold side of the part to be slick...when the resin saturated mat is placed on the form, gravity will cause the resin to flow to the mold surface. This results in a "slick" surface on that side. The engine compartment side of the part has less resin with more exposed fiber and irrgularities. The Evercoat Rage would be the right filler.

Original FRP body panels were press formed in heated matched male/female molds. When the heated mold halves were closed on the saturated mat, the resin was forced into fairly uniform distribution across the mat thickness resulting in smooth surfaces on both sides of the part.

[Chuck S.]

More On SMC Filling and Bonding...

Actually, I have read posts from experienced professional body guys saying they use polyester filler to smooth and contour SMC and other plastic panels. This makes sense if the surface of the part has been roughened with 80 grit paper where it's going to be filled. The filler doesn't carry any load...the adhesion only has to support the filler's own weight, which should be intentionally minimized. Structural bonding is where people get into trouble using the wrong products on SMC.

I don't think there is a special epoxy-based body filler for SMC and other plastic body panels...at least I've never seen one. If you read Lord Fusor's recommended procedure for cosmetic repair of SMC, they don't address the use of filler...they say to sand the adhesive to contour.

[Jim V.]

hand laid....?

Thanks again Chuck. Press molded is smooth on both sides as is this inner fender. If it is not SMC then it looks more like press molded to me. Ages ago I did some hand laid work for aircraft and one side is obviously very rough showing the texture of the fiberglass mat. The picture does not show well, but the motor side is flat with no rough fiber appearance. When sanded, it does show fiber strands.

Thanks...in either case I will go with the Evercoat Rage to fill the pock marks.

[Chuck S.]

Re: hand laid....?

Press molded is perfectly smooth...it doesn't have ANY "pock marks". Look at any other panel on your car...the back side may be rough near the bonding areas, but it's from sand blasting as prep for bonding, but generally, they are smooth as a baby's bottom.

Your inner fender is not REAL rough like virgin hand-laid because it's already been sanded by the perpetrator...the tops of the irregularities have been knocked off. He sanded until he was exhausted, and realizing he'd totally screwed up getting a hand-laid panel, he left the valleys.

I tried sanding a hand-laid panel smooth myself once, so I know what they look like half-way through. The disadvantage of sanding to get that press-molded perfectly smooth appearance is you also have a thinner panel once you get through sanding it away. If it's flat enough to make you think it's press-molded, then filling the pocks will be the best answer and will give you a stronger panel. Otherwise, you could always get a soft block and a random orbital sander and crank away on it some more.

[Chuck S.]

Re: hand laid....?

OK, you're right...I can't touch the panel in the photo. Maybe it is press molded.

If the car has never been repaired (there would be other ways of determining this), then maybe it is an original panel...one butt-ugly panel that GM let through quality control. Maybe the guy at General didn't pour in enough coffee cans of resin that day, and the top surface didn't quite have enough to make it smooth.

This of course brings up another point...if it's the original inner fender, then it would be over-restoration (and a waste of time) to fill the pocks.

[Wayne W.]

Re: hand laid....?

It is certainly one of the gelcoated reproductions. It should be polyester.

[Jim V.]

Prep for paint....

Thanks Wayne!

That makes sense to me. The wheel side appears as gelcoated with a high gloss waxy appearance. It all would fit that it is a early reproduction, circa 1985ish...

Now...what would you recommend in refinishing the wheelwell side? Just rough with 80 grit, apply 3M undercoating, and then blackout with Krylon 1613?????

As far as the motor side...fill with Evercoat rage, sand 200 grit, and Krylon 1613?

Thanks all for sharing your knowledge....

[Wayne W.]

Re: Prep for paint....

Thats about the only choice that you have short of replacing the innerfender.
It will be difficult to hide the sanding work on the engine side. The originals were bare fiberglass with some sand blasting. It will be hard to duplicate the fiberglass look there. Some speckling of primers and flat paint will help disguise it.

[Jim V.]

da pits....

Thanks again Wayne.

I have tried about 6 coats of DupliColor high build primer to no avail. Worthless for filling anything!

Next, I was going to try resin, filler, or glaze. Evercoat sells so many!! Which product would make the most sense....resin, Rage, Slick Sand, Featherfill...? I hope to apply via spreader or brush since I am not currently set up with spray equipment. I hope to finish with Krylon 1613 or SEM trim black.

Thanks....

[Wayne W.]

Re: da pits....

Use the easiest sanding filler that you can find, applied with a spreader.