Bottom Paint

[Peter McMinn]

Is there a difference between wet sanding sand paper and dry sanding sand paper?

Yes! Wet paper (black) and wet/dry paper (gray, generally) is put together to withstand water, which is needed to keep the abrasive side from "plugging", or filling up with sediment, which can create a dangerously uneven surface.

What grit are you using, or do you use more than one type to finish the job?

It really depends on what you are doing. If you are taking off a coat of paint, I would not sand, but rather use some kind of stripping compound. Once you are down to the gelcoat, or if you are fairing a painted hull, start with a grit no lower than 320. Keep the paper secured to a rubber pad that will conform to the shape of the hull, but maintain a uniform approach. Rub in steady circular patterns, washing out the paper in clean water every ten cycles. Be sure that the surface you are sanding is wet! I use a clean garden sprayer, but a hose would work fine. Important: gelcoat is actually pretty soft: no need for pressure. Instead, let the paper do the work. Raise the grit to 400, and then 600, 800 reversing the sanding pattern between grits. I used 1000 grit last spring, but I think that was over kill, as a good fiberlass buff and polish fills gaps nicely.

Would it be safe to use any kind of power sander? I have a small rechargable type that uses a triangular shape paper I believe.

No! Don't do it! You do not want to risk taking too much off. It would be difficult and nerve-racking to bring the original shape back. Use the rubber pad I suggested above. Go to West Marine or even Home Depot--you'll find them there.

Good luck!

[Roger]

Yes there are difference between wet and dry sandpaper. Without going into the actual chemical make up (because I don't know and don't have samples in front of me) the wet makes a bit of a slurry as you are working so you can easily see where you have been. It usually says on the package if it is for wet application. Depending on how rough your bottom is already, I would start at 120 grit on the smooth areas and go down to 80 grit to grind off some of the rougher areas, then move up to 180, then 320. You can go higher if you are not applying another coat of paint, up to 400 or 600. I have used up to 320 before applying varnish on a fg covered cedarstrip canoe.

If you do go to power, use nothing more aggressive than an ROS or palm sander. Definately stay away from a belt sander, although about half way through the job you are really tempted...

To keep your progress monitored, do 2 foot sections from centerline to gunwale moving towards one end.

[Guest]

Dry sandpaper is the standard old sandpaper you would use dry, for wood finishing,interior work etc., but if you get it wet, the sand falls off, and the backing turns mushy, like a wet paper bag. Whereas the so called wet stuff has the grit attached with a waterproof resin and the backing does'nt absorb water, but is more expensive.
In the cheaper dry papers, the grit is actually sand, better grades use Aluminum oxide granules, and harder grits are garnet which don't dull as fast.

John C. Jr. (ghampe-at-rcn.com)

[Guest]

Thanks for all your good advice. I will add this to my winter project list.

David Riley (driley-at-ngc.edu)