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Epoxy Coating..how many coats?


mjshp

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When GB says coat all surfaces with epoxy, how many coats?

What do you use to tell that the epoxy is ready for a second coat to insure primary bond and avoid saniding/washing the whole thing? Tacky but holds a fingerprint?

Thanks for your experience and ideas.

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10 mils is the usual "standard" to insure a piece of wood is fully encapsulated. This means it's able to keep the moisture content below the magic 17% threshold, which is where you can expect things to happen (rot, dimensional changes, etc.). This is the whole point of coating wood with epoxy, to physically wrap it in a plastic coating, thick enough to resist moisture vapor penetration that can cause issues.

3 coats is my general recommendation. Two will usually do, but any sanding or dry areas will be below the 10 mil threshold and you can have issues, so three coats insures you have enough.

If attaching a sheathing (cloth) you can use the "wetout" coat as one of the 3 you need for encapsulation.

If bonding a piece to a piece, then a single wetout coat, followed by a thickened adhesive mixture, will usually suffice. If you note any dry areas in the wetout, apply a touch more to that area, just to insure you don't resin starve the joint.

The "green stage" of the epoxy cure, is when you can apply a subsequent coat of epoxy (straight or thickened). This depends on formulation, temperature, humidity, etc., but generally, anytime from wet through dry to the touch, but you still can dent it with a fingernail is the safe range. This includes the tacky range and past it into the dry to the touch range. If it can dent with a moderate fingernail push, it's "green" and you don't have to sand or remove amine blush. Technically, you can push this further down the time line, but now you're guessing and this isn't good practice without some technical expertise and controlled conditions.

This said, some epoxy formulations will blush just by looking cross eyed at it during the cure, while others will not, unless you're trying to laminate in the rain. All amine cure epoxies will blush to some degree. Some formulations can resist this issue, in all but the worst conditions. These non-blush formulations are often more costly, but can be time savers. In the average back yard shop, conditions exisit where as you need to consider any cured epoxy suspect to surface contaminates, dust, blush, etc. so you have to clean and sand the surface anyway.

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Two at the least. Three is better. The first will soak in a bit and probably be a bit uneven. The second will coat everything well. The third puts some thickness to the coating. How long to wait is somewhat dependant on the epoxy being used. You can go for the second coat in just and hour or so. You just need to wait long enough so that you can tell that the epoxy is setting up. The third likewise wait at least an hour. More if you are using a very slow hardener or its cold. You can wait up to about 24 hours but again depending on the hardener that might be too long. I've mostly used the Bateau Marinepoxy with Medium Hardener. So long as the epoxy is tacky at all you are good.

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Would the same "tacky" rule hold true for applying primer and paints?

Applying paints to epoxy? Or each other?

Regardless, now you are talking about the chemistry of unknown compounds, ie unidentified paints. You have to be very specific here for anyone to offer advice, and you might want to check with the manufacturer of the paints as well. Most have good technical support on line for such questions. Some 2-part barrier coats recommend applying bottom paint within a window of partial cure to get a primary bond, but this does not mean it is a good idea for all paints and situations.

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I found out the hardway (demasted the mizzen) when my mast step failed, partially due to some two-part epoxy interlux primer around where I put the step. I had bare wood to bond but not enough, and I got lazy and didn't scrape off the two-part epoxy thinking it would bond ok. The bare wood part didn't fail actually tore wood off under bond, the epoxy paint area is where the tear started that took the whole thing off.

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