T-88 over System Three Clear Coat?

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Mike von S.

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Sorry if this has been discussed elsewhere, but I was not able to find a relevant thread.
I have taken over a FFP Celebrity project on which I intend to mount a Rotec 2800. I intend to strengthen the firewall bulkhead and side structure in the first bay to accommodate the weight and power of the Rotec. Unfortunately, the original builder used a varnish, System Three Clear Coat I think, over lumber I need to glue to. I will be using T-88 (made by the same company, I understand) for the structural additions.
How best to prep (or remove) the Clear Coat?
Will sanding the surface provide sufficient bonding of the T-88?
Should I try to remove the Clear Coat entirely?
Perhaps with MEK or another solvent?
Thanks
 
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Based on the MSDS sheets I suspect they are very compatible.
The epoxy base is the same with the clear coat having a diluent added to make it less viscous.
The hardeners are different, but of similar structures overall.

If you can't get any advice straight from System Three it may be worth testing some samples. Based on the claim that the clear coat is a "penetrating epoxy" I doubt sanding to bare wood is going to be practical.
 

TFF

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I have always suspected that Cear Coat is T-88 without thickeners. Someone on the Tailwind forum or the Biplane Forum had that question because they varnished the ply and wondered if it would stick to the ribs With T-88. System Three said there should be no issues. I have wondered about using the CC as glue it’s self.
 

rv7charlie

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I have wondered about using the CC as glue it’s self.
From the System 3 website:
'A two part epoxy coating and laminating resin '
and
'Clear Coat is ideal for clear coating wood, and the wetting out of fiberglass cloth for bright finished boats. Use as a build coat for clear linear polyurethane or varnish coatings and as a penetrating first coat for blister repair on fiberglass boats. Use also for those applications that require a long pot life and low viscosity. Use Clear Coat as the sole epoxy system to make wood strip canoes and kayaks. Protect Clear Coat from moisture during cure to avoid water spotting.'
 

TFF

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Frankly we use this stuff as a crossover from its real intent. I read all the stuff when used it years ago. Compared to West’s. Compared to other stuff. It a standard rabbit hole.

When you handle both T-88 and CC they are very similar. T88 is 1 to 1 by volume and CC is 2 to 1. That and the thickness seems to be the only differences. I used the CC as varnish which is supposed to be “cheaper” than the Polyfiber stuff. The CC is not thin like the Polyfiber varnish, which is water consistency. It might be thinner than T-88 but it is pretty thick. It’s about the same consistency as the bar top epoxy coatings. It does absorb into the wood some, so you can’t be stingy with it. So cheaper is relative. You get more volume per dollar with the CC but the Polyfiber goes further. Polyfiber is going to sink in more, the CC looks prettier. It’s really a wash. I did it, it works, but I would do the Polyfiber now. If you mix too much at once it will exotherm and gel some. Small batches at the ready instead of a bunch at once. The smell is better with the CC; that is a big win. If there was a certified epoxy for wood, no one would use anything else. So for wood airplanes, you need to be a believer. Do the voodoo dances, and shake charms. Pick your poison. I use the T-88 because it’s easy for wood. Fiberglass repairs I use West’s. The CC is pretty thick for laminating with fiberglass in my opinion. Maybe for fiberglass boats, but nothing delicate.
 

DanH

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Long time ago I spoke with System Three about bonding a leading edge plywood wrap to ply nose ribs. The rep said they were both bisphenol A epoxy. Not only were they entirely compatible, but they would happily co-cure...I could brush the inside of the ply wrap with ClearCoat, apply T-88 to the ribs, and strap it on tight for both to cure.

Back to the OP's question. I'd scuff sand the CC surface thoroughly, as I would for any epoxy bond to a previously cured epoxy surface.
 

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Gsport

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Clear Coat is basically thinned T-88; fully compatible in my experience, just roughen the surface. Used CC exclusively (instead of a 'resin') for making fiberglass layups. Takes a little longer to fully harden, but very easy to work with.
 

Mike von S.

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Sorry if this has been discussed elsewhere, but I was not able to find a relevant thread.
I have taken over a FFP Celebrity project on which I intend to mount a Rotec 2800. I intend to strengthen the firewall bulkhead and side structure in the first bay to accommodate the weight and power of the Rotec. Unfortunately, the original builder used a varnish, System Three Clear Coat I think, over lumber I need to glue to. I will be using T-88 (made by the same company, I understand) for the structural additions.
How best to prep (or remove) the Clear Coat?
Will sanding the surface provide sufficient bonding of the T-88?
Should I try to remove the Clear Coat entirely?
Perhaps with MEK or another solvent?
Thanks
Thanks to all for your suggestions.
This is from System Three's online Epoxy Book:
"[M]ake sure that the surfaces being glued are clean, free of grease, oil, wax, and other contaminants that could act as release agents. If the surface is coated with cured epoxy, sand before gluing and wipe the dust off. Prior to sanding wipe away any oil or grease with a clean rag and suitable solvent. Remove paint rather than trying to glue onto a painted surface. Epoxy resins stick well to sanded paint but the overall bond strength will be no better than the paint to substrate bond."
 

Mike von S.

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I received confirmation today from Brady Tinker at System Three Resins:
T-88 will bond tenaciously to cured, clean, and sanded (80 grit) Clear Coat. Provided good surface prep was done before applying the Clear Coat, in both cases the wood will fail before the bond line fails.
 
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