Patch technique

When I fiberglassed my Shearwater Sport, air bubbles formed over a few of the holes where I'd removed the wires, which, of course, spoiled the finish.  I sanded them down and patched them.  One of them bubbled up again.  Sanded down again.  But this time I applied a thin coat of expoxy first.  Hopefully that will fix the air bubble.  Now wondering if there is a better technique to make the patch.

Last time, I masked off the patch area with blue tape, laid cloth over it, and epoxied into the inside edge of the tape, waited a couple of hours, then trimmed at the edge of the tape with a razor knife and removed the tape and excess fiberglass.  That was neat, but left razor scores all around the patch.

I was trying to avoid frayed edges on the patch.  But the price was razor scores.  So best just to epoxy all the frayed edges and sand them down?

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RE: Patch technique

Use clear 2" wide plastic packing tape and epoxy to the middle of the tape. Once it cures so that it's no longer tacky but still flexible though stiff like leather (green stage), lift the glass gently off the tape and peel it back to the edge of the tape.

Nick the glass at the edge with a razor to start a tear and pull to continue the tear all the way across the glass. You need to pull strongly enough to peel right to the wood, but gently enough so as not to peel away from the wood. If the tear departs from the wood line just restart it with another razor nick.

Once you have all the glass off, let the patch fully cure, then hit the edge with a sanding block to get rid of the fuzz and to feather it into the rest of the glass. No frayed edges, no razor scores.



RE: Patch technique

Laszlo - Thanks for your reply.  Just to make sure I've got it, you're saying that once you make that first nick, you can tear a more-or-less straight line, at least for a while, or do you end up nicking it pretty much the whole way around?  Do you remove the tape after trimming the glass?  And then, put a couple more coats to fill the weave before sanding the edges smooth?

Thanks for taking the time to reply.

RE: Patch technique

Yes, when the epoxy is at the green cure stage a tear will tend to continue in a straight line if done correctly. So make just one single nick at the edge of the cloth such that several consecutive up/down threads are severed and the epoxy is sliced all the way through. Now when you pull, the slice will act as a stress concentrator and cause the rest of the up/down threads to break while the horizontal threads will be unaffected making the tear propagate horizontally. Mind you, the real world will intrude and inhomogenieties in the thickness of the epoxy, the threads, etc. may make the tear deviate, but if that happens, grab your razor and restart it.

Here's some more pictures:

Glass peeled up off the packing tape.

Glass torn along edge of tape. You can see the packing tape below the torn glass. You can also see where the tear started to deviate, so I stopped tearing and restarted the tear by cutting a new nick into the glass at the proper place.

Peeling done, tape removed and edge lightly sanded to get rid of the fuzz. Ready for the fill coat and final sanding.

Merry Christmas,


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