grumman41

My plan is to gelcoat my 1971 Vette....I was prepping my t-tops for gelcoat and I think one is SMC. Its my understanding that most gelcoats and SMC panels are not compatable, what are some good options? Simply find another panel or use a different sealer on one t-top?

There are some year numbers molded into the top, they range from 71 to like 74, will this help to determine what year top this is? I was thinking it was SMC by the color of the cover tabs(later years), marbling of the finish and it sands different than my other panels.



Thanks, Kevin

DUB

Kevin,
I honestly have asked this SAME question and wondered WHY ....as 'they' say....gelcoat can not be used on SMC. BUT...when I talk to tech guys...I ask them....then IF gelcoat is polyester....much like about every body filler that can be applied to SMC....and polyester primer is OBVIOUSLY polyester like gelcoat.....and the polyester primer CAN be applied to SMC......then WHY can't gelcoat be applied. I still have not gotten an answer that satisfies my curiosity.

SO to answer your question...you can use polyester primer....which is close to gelcoat...but still not the same as gelcoat due to the talc in it that is what is the filler to allow it to build so well.

YES...as a basic rule...a light gray panel with marbling is an SMC....so to speak.

DUB

grumman41

Thanks DUB. The t-top panel that I think is SMC sands much easier than every other panel on my car.....does most SMC panels sand different than the "old" panels?

I also don't see the fiberglass hairs as much.

Kevin

DUB

To me... they sand about the same. Depending on the grit I am sanding with. I have never really given 'how the panels sand' any thought. All I am concerned about is that they are sanded sand ready for whatever I plan on applying on them.

SMC...you really should NOT see any hairs...so-to-speak. You will see the marble effect in some areas. The 'old' fiberglass you can see hairs that look like cooked spaghetti on a plate....but obviously much smaller in diameter.

If you are still concerned...post photos of both T-tops.

DUB

grumman41

To "close this thread out", I spoke with the senior tech at Evercoat. He said the Evercoat laminating gelcoat (the type used with PVA) would be fine on the SMC panel. He gave me some sanding and cleaning info but it was standard procedure for prep.

This is different from what I have heard from many experts in the field. I'll let you know in 30 years if its correct info.

Kevin

DUB

To open it back up.

This is the SAME discussion I have had with many chemists....many , many times. IF a polyester product can be applied to SMC and bond...then why not a gelcoat....and if a polyester primer can be shot on SMC....then why can't gelcoat....and why is it that when I have used a polyester resin on NEW SMC...it had delaminated and came off with little to no effort. Along with one brand of SMC resin. The 'stuff' didn't stick at all. But if I apply it to an early generation SMC...the polyester resin sticks....or at least appears to do so.

One thing that I have come to conclusion on and can never get a straight answer is that the first/early generations of 'SMC' are NOT like the SMC of today. But I know they have to be different.

SO...knowing when the 'blends' of what made SMC...and when it changed is something that I do not know for a fact.

Hopefully you have it handled.

DUB

grumman41

Thanks DUB, At least my issues will be limited to the t-tops. This would be a relatively easy fix compared to other parts of the body. To keep this project moving, sometimes you have to do your research and then move forward.

It was interesting, to me, that the tech recommended sanding with 120 to 220 grit and wiping with acetone. You would think, if adhesion might be an issue that 80 or 100 grit might be recommended?

Kevin

DUB

ALWAYS A VERY WISE move!!!!! MANY people do not do that and then wonder what screwed up and hope for an 'easy fix'.

YES to using acetone.

ALWAYS thinking that the ROUGHER the grit sandpaper being used is ALWAYS better. IT IS NOT.

It all depends in what is going to be applied to the sanded surface.

So...the tech is correct (obviously)...because I would use 180 grit...which falls in line with being between the 120-220 grit recommendation.

This is where 'things ' can get 'funny' and all screwed up. many companies develop and test product and have a preference in what the surface should be prepped to. BECAUSE...if you were to magnify and look at the sanded areas from the side. Lets say for an example...36 grit would look like the saw teeth on a 'pioneer days' 2 man buck saw.....80 grit would look like a hand 'rip' saw. 180 grit would look like a coping saw.....320 would look like a 18 tooth hack saw blade.

Each example above...the depth and width of the grooves are different. AND...much like a fine threaded bolt....the finer you go...the more surface area.

SO...DEPENDING on the product...and its viscosity....SOMETIMES going finer is BETTER....and then sometimes it is NOT.

SO...it all boils down to doing what you did and that is RESEARCH and talk with the people that know the product.

Becasue...when I go to bond on a new SMC panel using the two-part adhesives. I DO NOT grind the panel where I am bonding it. It gets scuffed...which is better. BUT...if I were bonding on a fiberglass panel.....I either grind it well or sandblast it. So...there lies a contradiction in what works for one product...does not necessarily work another product....that and YEARS of messing with this stuff and finding out what works.

DUB