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Working wet on wet with epoxy

“Wet on Wet” working with epoxy.

If you encountered the term “wet on wet” and would like a detailed description of what that means, then you have come to the right place. Working “wet on wet” in very simple terms means application of epoxy while the previous application of epoxy is still curing. How long to wait for different scenarios is the tricking part we will explore here.

Working wet on wet results in better bonds (chemical curing between layers), less epoxy use, and less sanding work.

Epoxy will chemically cure (primary bond) between layers if you apply subsequent layers within a window of time. That window of time is dependent on the working temperatures and the speed of hardener you are using. Ideally, the next layers are applied just as soon as the first layer is tack free, but you can achieve a chemical bond for at least 12 hours (longer for slower cures).

When applying more than one layer of fiberglass: Working wet on wet will save epoxy use. Excess epoxy from the first layer will be absorbed by the second layer. Also (more importantly) laying down the second layer while the first layer is still wet will save you the time a sanding between layers. Stronger, lighter, cheaper, less time!

Apply fiberglass over “wet” epoxy fillets. When joining two parts with a fiberglass joint, such as frames, stringers, bulkheads, etc you should apply your fiberglass while the fillet (radius of epoxy putty) is still “green”. This will not only allow the epoxy fillet to cure along with the glass, but it will also allow the fiberglass to lay into the fillet without any air bubbles. If you apply a fillet and let it cure, you will need to come back and sand the fillet in order to roughen it up for a bond and to remove small “ticks” of putty that will keep glass from laying flat.

Multiple layers of epoxy for a build up: When encapsulating wood, you can apply multiple layers of epoxy and only wait as long as it takes for the first layer to become non-sagging or almost tack free. If you apply too early you run the risk of pulling up the first layers. There are some tricks to building up a clear epoxy finish, see our other tutorial for tips.

So what happens if you miss the “wet window”? Epoxy resins and glues give very good mechanical bonds (secondary), but you have to do a little prep work before going to the next layer of epoxy if your first layer has cured. The preparation is basically the same no matter what epoxy you are using; the surface must be free of contaminates (amine residue) and it must have enough “tooth” for the epoxy to latch onto. The more “tooth” the better the epoxy can bond. If you are working with an epoxy that has the potential to blush, then you must wipe the surface down with a solvent to remove the blush. You can also wash the surface with soap and warm water, but I have never found that to be a very good option myself. You remove blush before sanding, or you may just be moving the surface contaminate around or grinding it into the surface.

Saving time and material by working wet on wet also applies to primers and paints, but thats for another thread.

Over the years I have used dozens of different epoxies for boat building and repair. I am very happy to offer for sale the products I use myself at ------ “We use it, we love it, we sell it, in that order”

If you have any questions on this subject or other boat building, boat repair, or general composites, please post them at our tech support site and we will be happy to help