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Thread: Wet Sanding and wood swell - Opinions?

  1. #1

    Wet Sanding and wood swell - Opinions?

    Hi,

    I have a basswood guitar body and it is almost time to commence wet sanding of my clear coat.

    However, I've read online that water can cause unpainted areas of the guitar to swell and overtime, cause damage. Eg. around areas such as bridge post holes, pickup pockets etc.

    Is this true? What are people's experiences? Could you substitute water for another liquid to lubricate sanding? Would a mineral or plant based oil be OK?

    Cheers.

  2. #2
    Overlord of Music Sonic Mountain's Avatar
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    You just need to be extremely careful not to let the water get onto any unpainted areas. It will swell the wood and wreck your finish. I normally have a towel handy and sand a bit then immediately dry it off. You can also block off holes with toothpicks or similar which helps a bit. It is also possible to just dry sand with a very fine grit paper - but you'll use a lot of sandpaper as it will clog pretty quickly. If you use a decent cutting compound prior to polishing it can remove the need to spend too much time sanding flat, but you'll need an electic buffer of some kind to get the best results.
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  3. #3
    OK thanks....So I'll need to be extremely careful then!

  4. #4
    Here's a couple other options I use.

    1) "painting" shellac into the holes with a small artists brush (5-6mm one, not the teeny-tiny ones). 3-4 coats or until you see some build-up. It dries really quickly so you should be able to get it done one day and wet sand the next.
    get the sides as thoroughly as you can without getting it on the finish on top. If you do, you can wipe it away with bit of rag if you get to it quickly, and a tiny bit will be covered by pickup rings etc. This works particularly well with the large holes like pickup route and control cavities.

    2) plug the holes (like the bridge post holes) with non-absorbent material. After I pack the hole (I use balled up painters tape) I make a little "cap" by cutting the painters tape to fit over the top of the packed material so it goes as close to the edge of the hole as possible. doesn't need to be perfect, but best as.
    You can wet sand right over the plugs without catching them with the sandpaper.
    Of course you can also combine both these methods.

    And like Sonic said, keep a dry rag/towel handy to mop up as you go so the water doesn't puddle anywhere for too long.

    I presume you're talking about the RC-1 Custom kit you were/are building. If that's the case, one thing I noticed in your build diary, is you masked off the pickup routes when you painted. When I spray the finish, I spray right over (into) the pickup routes. This seals them and prevents having to worry about water problem when wet sanding.
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  5. #5
    Mentor fender3x's Avatar
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    I have heard that some people wet-sand using alcohol rather than water because it evaporates much more quickly and doesn't swell the wood as much. I am nervous about using it with a finish, however. Poly is pretty had to damage, but alcohol is a pretty good solvent...

  6. #6
    Mentor fender3x's Avatar
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    ...So I tried to do a little wet sanding using isopropyl alcohol rather than water on acrylic poly...my test piece that had been curing for a bit over a week. I don't think it caused the finish to dissolve, which is the good news, but it made the material it removed clump. Won't use it again over finish. It did not do any real damage, but I am still glad to have tried it on a test piece first.

  7. #7
    I've been trying to find a decent solution to this problem for ages. All the suggestions above are pretty good in many cases but I'm still searching for the "perfect" solution which probably doesn't exist but I'll keep searching.
    My latest build is a 4-string bass and there are no surrounds for the pickups and no pick-guard to hide any mistakes around the control knobs, etc. The holes for the control knobs I plugged with bluetack from behind and then indented them slightly with my finger tip. That was before I primed the raw timber. By the time I had the topcoat done there was nowhere for water to seep into the holes. I taped over the screw holes for the bridge and went as close as I could sanding with a tiny bit of paper and my fingers. The bridge will cover the part I couldn't sand. The pickup cavities are the only problem. Haven't been able to avoid some swelling around one of the corners no matter how careful I was. But then I touched that up with an airbrush and it didn't need any sanding because the airbrush finish was already smooth. Maybe I should do the whole body with the airbrush.
    I heard naphtha can be used and/or mineral spirits but I haven't been game enough to try either. They might be the same as isopropyl but I'll let somebody else try first.

  8. #8
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    You don't need to use a lot of water to wet sand. Get yourself a spray bottle, slightly misted the surface, stand your sanding, and keep wiping of with a rag, and continue the process.

  9. #9
    Member JimC's Avatar
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    I suggest that its also important to wash the dreck off the sandpaper regularly. That both keeps the paper wet enough for the cut to work and keeps it cutting cleanly.

    But most of all, try and make sure there are no bare areas. Brush round holes with a bit of varnish, top coat, whatever, it hardly matters, and you can just ease them back out to full size with a drill or file when you're done if they end up a tad too small with the extra coating.
    Last edited by JimC; 10-10-2019 at 07:27 PM.

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