View Full Version : Sanding through the glass

08-08-2012, 10:06 AM
If I sand through the glass and patch it later with a piece of cloth and epoxy is there any way to make it look ok or will it really stick out when finished? I'm building a cedar strip canoe with west 105/207 and 6oz cloth.

08-08-2012, 10:09 AM
It will look 'ok' and won't really stick out, provided you sand the patch meticulously flush and smooth.

David G
08-08-2012, 10:12 AM
Two thoughts: you can patch it, and how bad it looks depends upon your skill and your level of critical judgement; you can not patch it if it's not in a critical spot.

So... if it's in an area where the glass isn't critical for strength or abrasion resistance... you can just level the area with neat epoxy or thickened (only if you'll be painting the area) epoxy, sand it off flush and paint/varnish.

Or, you can do a repair - using a variety of techniques - depending upon how important the area is strength-wise, and how critical you're gonna be of the final product aesthetically. A bit more info from you would help. Fotos are always useful.

08-08-2012, 11:56 AM
The finer the grit paper that you use before you overcoat the epoxy/glass, the less visible it is when you overcoat.

If you've gone through a structural area, by all means reinforce it with a patch of cloth.

+1 on images being very helpful.

08-08-2012, 11:57 AM
This has probably been asked and answered a million times elsewhere on the forum, but what's one more, amirite? And my question is, how far down should one sand? My understanding is that the epoxy should look "cloudy" but you're not actually trying to get down to the fiberglass. Sanding-mania is tonight so I am quadruple checking my research before having to check in to how to fix what I did wrong.

Todd Bradshaw
08-08-2012, 11:58 AM
What are you sanding with? Unless it's a big honking disk sander, you should have plenty of warning before you go through the glass. The second you have gone through the filler coats and start to cut into the glass, you will see a regular pattern of small white dots appear. These are the peaks of the yarns of the fiberglass cloth. If you see them, you have already gone deeper than ideal and should immediately stop sanding that spot. If you continue, the dots will turn into an obvious woven fabric pattern as you cut deeper. By this time, the area is starting to seriously lose strength. Going even deeper will eventually go all the way through the cloth and most likely sand a flat spot on the wood. Glass is much harder than the wood is, and just as soon as a sander gets through the fiberglass, that soft wood core cuts like butter - and it happens so fast that you're screwed before you even know what's happening.

Other types of sanders (orbitals, random-orbits, big disk sanders with a feathering disk, hand sanding with a long-board, or even air-files) have much less chance of cutting too deep because they are slower and usually use finer grits. I find no use whatsoever for belt sanders on strip canoe hulls and in many cases, they may guarantee you a lousy sanding job. They're flat with sharp edges, canoes aren't. If whatever you are using is cutting so deeply that you're seeing cloth weave, you need a different sander, finer paper and/or a much lighter touch with the sander.

A proper fix of a spot that is sanded through would be to very carefully feather out the glass around the hole, typically with maybe a 1.5" to 2" taper on all sides. Then you check the wood (eyeball) to make sure there isn't a dent or flat spot there which would need filling. Then you lay on the new cloth and epoxy, oversized to cover the entire feathered area with the weave direction matching the original cloth as well as possible. It should probably get at least two filler coats as soon as it's hardened enough to support them (this avoids getting amine blush down in the weave where it's harder to remove). Let it all harden a few days so that you can feather out the patch with careful sanding. You will be cutting into the weave around the edges of the patch as you sand, but there is no other way to taper the patch out. When you think the surface is fair, wet it down with a sponge and water and eyeball it from several directions. You're looking to be sure your patch is blended into the hull shape and not making a bulge. Then I'd add a couple more filler coats over the patch area and give it a final sanding to blend them into the surrounding surface. It's a lot of tedious work, but if you want to do it properly, that's the way you do it. Obviously, it's a far better idea to sand carefully the first time and not have to do it at all.

08-08-2012, 03:05 PM
I rushed my bias strips and one has a bad wrinkle that is about 3/16" proud of the rest of the glass. I am wondering would it be better to sand that wrinkled glass flat, right through the cloth, or to build up around it?

Todd Bradshaw
08-08-2012, 03:15 PM
I'd cut the wrinkle off (knife blade, sanding, Dremel or whatever else can do a clean, neat job without much impact on what's around it). Maybe add another strip over the top if you think it needs the structural strength there, or possibly just fill that area. It's hard to say more without seeing the boat.

08-08-2012, 05:10 PM
...My understanding is that the epoxy should look "cloudy" but you're not actually trying to get down to the fiberglass. ...
You understand correctly. The idea is to sand the surface gloss so it is no longer glossy. Cloudy is as good a descriptive term as any. Once the smooth gloss surface is gone sand no farther or you're just sanding away precious epoxy and risking going through to the cloth. I'm in the habit of wash, sand, and wash again, to make sure there's no hint of blush remaining.

Todd Bradshaw
08-08-2012, 07:14 PM
Perhaps "frosted" would be a better term than cloudy as the change is all right at/on the surface. The epoxy hasn't become cloudy. It's simply a matter of its surface being uniformly covered with tiny scratches which obscure its transparency.