Wet versus dry sanding

December 10, 2005 1:44 PM

What is the general rule as to when should I use wet sanding versus dry sanding?
posted by bobbeene to home & garden (3 comments total)

sand "papers" that bond the grit to the paper with glue don't work when wet. Water degrades the paper quickly, and it will also debond the girt. You end up with a layer of grit on the surface to be sanded, rather than a smooth surface. Sanding "films" either are just a substrate with a rough surface, or are a substrate with the grit material bonded in a way that is not water soluable.
posted by b1tr0t at 1:52 PM on December 10, 2005

The answer to that depends somewhat on what kind of finishes you're dealing with. Paint on wood? Clear finish on wood? Auto paint?

For example, high-end interior painters often wet-sand before painting and between coats. The reason is that the sanding dust stays wet and doesn't get into the air to ruin the smooth paint later on. A benefit is that the sandpaper lasts a long time; you just keep rinsing it in water.

I thought it was a crazy idea fit only for obsessives -- but I tried it and now swear by it for prepping interior trim.

Also, latex paint doesn't sand well. If you sand it wet, the sandpaper doesn't get clogged as quickly.

Years ago, I'd heard only wet sanding only for smoothing clear wood finishes when fine quality is desired. When you've built up enough thickness with the varnish, polyurethane, or drying oil, you use very fine wet/dry sandpaper (600) to smooth off little imperfections and dust flaws. Water make scratches less visible and gives you better control to achieve a uniform thickness of finish. Using oil when sanding does the same thing, and can leave you with a satiny finish that doesn't need recoating.

And of course, keeping dust out of the air is extra-important when clear-finishing wood.

I know that auto-painting has its own uses for wet sanding, but that's beyond the scope of my experience.

By the way, if you want to wet-sand, you need to use wet/dry sandpaper, not conventional sandpaper. The black sandpaper you commonly see in hard-ware and paint stores is usually the wet/dry variety.
posted by wryly at 2:08 PM on December 10, 2005

In general, the more poreous the material, the less likely I'll wet-sand. Wet-sanding is preferable only because it keeps the dust out from in between the grits (reducing the effectiveness of the paper/block). But in situations with poreous materials like wood, the extra moisture will cause all sorts of problems. Mainly it gunks up the wood and roughs up the finish. For metalwork, I always wet-sand. I recently polished my valve cover for my car (which is just sanding, sanding, and more sanding), and wet-sanding probably saved me 10 hours (of a 30-hour job).
posted by Civil_Disobedient at 7:09 AM on December 11, 2005