Wooden Boat People

By McKenzieDriftBoat.com

An (overly) simple question re: Epoxy and Sanding

As I've posted before, I am pretty new to boat work. I am restoring a craigslist rescue boat. The inside is cleaned up and ready to go. I just flipped the boat. The bottom is in relatively great shape. It appears to already have 20 oz fiberglass on the bottom, with a few coats of epoxy. The epoxy in some places is a bit messy where the previous owner might have done some repair work. . but that can be cleaned up.

I was going to, for good measure, clean up and put a few new coats of epoxy with graphite just to clean things up and get it good and coated.

Question: I am not entirely sure how much sanding to do on old epoxy before applying new epoxy. Is it as simple as a quick rough up? Or more than that? I've searched throughout previous conversations but still feel I am not entirely sure how much sanding to do.

Thanks

sam

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Replies to This Discussion

Permalink Reply by Guy Fredrickson on June 25, 2012 at 4:33am

Sam,
I think that you want to avoid sanding through the underlying layer of fiberglass cloth. It doesn't matter how much epoxy you remove by sanding, because you can replace the epoxy by simply applying coats. But once you sand away the fiberglass cloth, it's gone (unless you go to the trouble of laying down another layer of cloth).
You should be able to tell when, while sanding, you reach the fiberglass. You should be able to detect the weave of the cloth. I'd stop there unless there was good reason to sand through the cloth to the bare wood.
I hope that helps.
Guy

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Permalink Reply by Phillip Westendorf on June 25, 2012 at 6:54am

Sam,

I've repaired a glassed stripper canoe (it hit a couple of big rocks!) and had to repair it and did exactly as Guy has said. I added thickened epoxy (cabosil & wood dust) to fill in some deep gouges (to the wood). In those places I added small patches of glass cloth and added the normal four coats of epoxy to buiild it up and then sanded it fair again. Added another 4-5 coats of Varnish too. Wasn't too difficult, just time consuming.

G'Luck,

phil w.

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Permalink Reply by Sam Snyder on June 25, 2012 at 8:20am

Thanks Guy and Phillip,

That helps. Another question: Do I need to sand that far? Do I need to take all the epoxy off, down to the weave? Or, can I just focus on getting the bottom smooth? Not to sound lazy, but what is the minimum sanding needed to epoxy before adding another layer to it?

Thanks again,

Sam

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Permalink Reply by Rick Newman on June 25, 2012 at 10:44am

Sam, after about a week or more the epoxy achieves it's final toughness. With most epoxies you have a window of 72 hours in which to make a chemical bond between the older and newer epoxy layers. After that most manufacturers recommend that the surface be roughed up with 80 grit abrasive to provide a mechanical "tooth" for the new epoxy to connect to.

So unless you enjoy sanding epoxy while wearing personal protective gear that's all you need to do. If the previous repairs have left an "unfair" surface you can flatten the repairs to achieve a "faired" or smoothly flowing surface.

The only trouble comes when you damage the fiberglass that was laminated to your boat. The fiberglass achieves it's strength from the continuity of the fibers and once they are cut or damaged the strength diminishes. You're headed down the right track. We would love to see some pictures. If you find the posting process difficult let me know I can help you out.