Proper bonding of subsequent epoxy coats

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    Diving Duck

    [I have submitted the following letter to CLC.  When I get the answer I’ll publish it as a reply to this post.]

    I’m starting to sand the shell now for ‘glassing it and was wondering if the plan is to apply subsequent coats during “tack” or at least during the “thumbprint” stage–apparently not. I’m no expert on epoxy, but I have built three of your boats and have studied Russell Brown’s book, “Epoxy Basics.” (Admittedly that is about West System products but many of the principles are the same.) I have also read through MAS Epoxies’ FAQs several times AND checked recently to see if they had changed the procedures, but they have not (or if they have they have not updated the FAQs).

    My understanding is that the best possible bond is a chemical bond, which can be achieved during the “tack” phase of curing, when the epoxy is starting to “kick” but is still tacky enough to pull a few strands off a cotton ball. The next-best is a mechanical/chemical bond which you can get during the “rule of thumb,” i.e., the epoxy is no longer tacky but you can still make a thumbprint in it. A light scuff sand is recommended here, but my feeling is that if this is the second coat you should use purple Scotchbrite pads to minimize damage to the fiberglass cloth.

    In the manual, though, you would have us wait 24 hours, well beyond “rule of thumb” or tack. There is no mention of scuff-sanding for either of the two subsequent coats.

    Has MAS changed their formula and not told anyone? Or, since this is not a boat, are you less concerned about the layer bonding? Again, it would be much appreciated if you can write me back by Friday. If you can’t manage it, I’ll go ahead and follow my old method. It can’t hurt.

    Diving Duck

    Here is the (very quick) answer from John Harris, which definitely takes a load off my mind:

    We’ve always done it this way, and MAS is still a blush-free epoxy so you can go a few days before overcoating.  That’s certainly how our Teardrop was built, as it was never convenient to apply that next coat at the perfect window in the cure cycle.


    Chemical bonds ARE better, no disputing that.  But I think it’s a case of needing a 200%-strength application, where waiting until the epoxy cures gives you 400% and a chemical bond gives you 500%.  We were good at 200%, so all the extra work to get to 500% isn’t really worth the trouble.


    There are exceptions. Someone building in a carport in Seattle is going have trouble with fog and rain causing the no-blush MAS epoxy to blush, so they’d have to proceed more along WEST System guidelines.

    The rest of us are okay.

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