Re: Epoxy adhesion

Posted by CLC on Apr 13, 2005

>>>>>>>>>I had adhesion problems between the second and third coats. I think it's because I let the second coat cure and didn't sand.


Random thoughts on a complex topic. At issue is "secondary bonding," or what happens when you put new epoxy over old epoxy, and how old is "old."

The non-blushing epoxies (MAS, System 3's Silvertip, System CLC, to name a few) are supposed to allow you to put on subsequent coats of epoxy at any time. Blushing systems (which includes most epoxies used with Fast hardener) create a greasy film on the surface as they cure called "amine blush."

Non-blushing epoxy was always incredibly important to us at CLC, as the pattern of almost all garage boatbuilding projects is that you work on the project at odd intervals as you find time. Either being locked into an 8-hour "green" cycle of recoating (catching the epoxy before it has blushed heavily), or having to scrub off amine blush every time you come out to work on the boat is simply impractical for most people.

So in theory the non-blushing systems let you come back to the project whenever you like without worrying about the glue curing, and in practice that's usually the case. But it's never so simple; if it's especially humid, for example, you can get a greasy film on the surface as the epoxy cures and subsequent coats will give trouble unless you carefully wipe everything down with denatured alcohol or lacquer thinner.

Other things in the air---silicone spray from the garage door repairman, fabric softener-laden exhaust from a clothes dryer, to cite many examples that I've encountered---can conspire to contaminate the surface and defeat the next coat of epoxy. If in doubt, a wipe-down with solvent in your well-ventilated space will erase doubts.

How long can you wait between coats of epoxy? With the no-blushing systems, here's a rule of thumb: Within 24 hours, no problem. Roll on. Within 3 days, use caution; read the above and make sure that no surface contaminants have gotten at it. Within a week, it'd be wise to wipe down the hull with solvent before you recoat. Six months (don't laugh, it happens in our shop all the time): definitely clean up with solvent, and maybe think about sanding to key up the surface.

In Response to: Epoxy adhesion by Jon Zeidler on Apr 11, 2005