New tool eliminates noxious epoxy sanding dust

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cedarphile
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Joined: Sun Jan 22, 2017 11:41 am

New tool eliminates noxious epoxy sanding dust

Post by cedarphile » Wed Jan 02, 2019 3:23 am

I think the worst job in building a cedarstrip canoe is dry sanding the epoxy coating on the hull exterior to get a smooth, fair, ripple-free surface in preparation for brushing on spar varnish or spraying 2-part polyurethane clearcoat. So I've switched to wet sanding the epoxy on my hull exteriors and found the process safer, faster, and cleaner. Now a new tool speeds up that process and produces a perfectly fair, ripple-free hull.
auto parts store sander.jpg
Auto parts stores have a 1/3 sheet flexible sander used in body shops that's perfect for wet sanding epoxy on canoes. The sanders cost about five bucks.
flexible sander.jpg
By pushing on the handles, the sander conforms perfectly to all of the various curves of a hull. You can use up-and-down strokes, diagonal strokes, longitudinal strokes and if you keep pushing, it always conforms to the hull. For the final pass, I hold the sander diagonally to the curve of the hull and move the sander the same direction as the strips.

I start with 120 grit wet/dry 3M Sandblaster paper and only use 2-3 of the 1/3 strip pieces on one half of the hull. I can't explain why wet sanding makes sandpaper last so long, but it does. I keep an ice cream pail of water with a few drops of dishwater detergent on the hull so it's always handy to wet sections of the hull and to sponge off the wet slurry once a section is sanded.
sander, sponge, pail.png
If your starting surface is particularly rough with runs and drips because you didn't use the brushing techniquesTed Moores details in Canoecraft with warmed epoxy in small batches so it flows out well, you may have to begin with 80 grit and switch to 120 when the worst of the boogles are gone.

Expect to have a couple gallons of liquid on the shop floor when done, so plan for a way to handle that.

Depending on which auto parts store you buy from, the commercial sander you get may be too stiff to comfortably curve it enough so the sandpaper contacts the hull along the entire sander. If you can't find a store with a more flexible one, you can make your own from a piece of 9" x 2-3/4" thin plywood cut from an old hollow-core door, two 1x1s and two handles made from 3/4" dowels. Cut the plywood piece so the 9" dimension is cross grain so it will bend the best. If you cut the piece with the grain, some plywoods, particularly luan bend poorly and may break when you try to conform it to a tight curve. I've found oak door-skin plywood the most flexible. Run a long screw through each 2 3/4" 1x1 and into the dowel to hold the handles solidly. You can then glue the 1/1–dowel assembly onto the ends of the plywood with waterproof glue, or you can use screws with the heads countersunk. The sandpaper is held on the sander with duct tape on the ends of the strip and the blocks.
Homemade flexible sander.png
In practice, you can't use too much water and if the slurry on a section you're sanding starts to get dry and "growls", splash on more water or wipe off the slurry, dribble on more water, and keep sanding. Periodically, dip your sander in the water to clean the paper, and change the water 2-3 times to do one half of a hull.

If all the runs and fisheyes are gone when you finish with the 120 grit, and you've made your final passes inline with the strips so sanding scratches are inline with the strips, congratulations, you'e done. Varnish or clearcoat will fill the scratches and give a perfect surface. I'll sometimes go over the hull lightly a second time with 150 or 220 grit wet/dry paper to eliminate the tiniest of ripples with the "flexible longboard" but that's rarely necessary.

Not only does wet sanding with a flexible sander yield a perfectly fair hull with no dips or unfair ripples, it's kinder on the lungs. If you dry sand epoxy even with the best of masks and with a Shop Vac attached to your sander, that fine epoxy/fiberglass dust is still everywhere in your shop when you take the mask off. It's being stirred up regularly as you move around your shop and relaunched into the air and into your lungs. We don't know the long-term lung hazards of epoxy dust because we've only been sanding epoxy on cedarstrips for a few decades. A few decades down the road, will we see sleazy lawyers–who now seek patients with asbestosis and mesothelioma for tort actions–seeking patients with epoxyosis?
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Cruiser
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Joined: Sat Apr 27, 2013 10:21 am
Location: Bowmanville, Ontario

Re: New tool eliminates noxious epoxy sanding dust

Post by Cruiser » Fri Jan 04, 2019 9:32 am

Good info on using wet sanding .... epoxy dust does tend to be very mobile, although last time I was forced to do the operation in my shop (usually I head outdoors) because I was working in the winter. I have a shop vac attachment for my ROS and I couldn't actually see any escaped epoxy dust when I finished ... I am sure there was some and I still wore my mask, but the vac attachments can work really well.

A point about drips and runs, IMO you are much better off not trying to sand them, go over the hull first and use a carbide scraper to get those area flattened before starting your sanding process.

I used a block on my last build, the contour sanders I looked at were just too expensive, I should have looked at an autobody supplier, I will definitely look at trying that.

Those pics are really good, but I think they have been resized and come up like thumbnails, maybe a little higher res for bigger pics would be nice, the details get sort of lost that small IMO .... but this is good info I will likley use in the future.


Brian

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