Will a WB finish warp my control cavity cover.....

Discussion in 'Luthier's Corner' started by honza992, Apr 28, 2016.

  1. Hi all

    I've only done a wooden control cavity cover once before and it wasn't a success. Using Minwax Wipe-on Poly (or Tru-Oil, I now can't remember which) warped the cover significantly, I think because I only applied the finish to one side.

    The next finish will be a water based urethane (Enduro-VAR from General Finishes).

    I can see three possible solutions:

    1. Spray both sides at the same time. Hopefully the expansion of the wood will be symmetrical and the cover will stay true.

    2. Stick the cover to something flat to stop it from warping, then spray just one side.

    3. Another technique that I haven't thought of!

    How does everyone else finish their control cavity covers?

    Thanks as ever for your help and insight.
  2. earlysecond

    earlysecond In Memoriam

    Jan 26, 2016
    Do you have the capability to spray vs brush it?
  3. Yes my plan is to spray, HVLP.
  4. A few of my early basses have warped control cavity covers. I make them a bit thicker now, that seems to do the trick. I have always put finish on both sides of the cover.
  5. earlysecond

    earlysecond In Memoriam

    Jan 26, 2016
    If you are spraying, I would hang it vertically and your lightly dust the first 3 coats. Approach them more as a sealer than a finish coat. Although I cannot prove it, I am thinking that you should spray both sides to seal then after that is accomplished, you can pound some clear on the needed finished side of the cover. Think through this with me. The solvent, water in this case, is soaking into the wood and causing it to warp. The less water applied in a single coat the better. This is my rationale for sealing the piece first. After I seal then I think I would level sand the finish side.

    Can you practice on some waste material? That is how I learn the most!

    Hope that helps,
  6. Manton Customs

    Manton Customs UK Luthier

    Jan 31, 2014
    Shropshire, UK
    Luthier, Manton Customs
    I'd say @Jisch has the right idea with making them a bit thicker. I can't see the water in a water based lacquer being too much of an issue, any wood I work I always raise the grain (with water) which causes the wood to get considerably wetter than it does from the first few coats of WB.

    Another option is to laminate a piece of aluminum or plastic with your chosen veneer.
  7. tjclem

    tjclem Commercial User

    Jun 6, 2004
    Central Florida
    Owner and builder Clementbass
    I make min 6-5mm thick. with no warp problems
  8. Thanks all. This one is only 3mm so if it warps I'll re-rout the cavity lip for a 6 mm cover.

    I'll try some very light coats of finish on both sides to begin with and hopefully that will seal the wood sufficiently for it not to be a problem.
  9. You can also make one that is 6mm, then trim the edge that's on the inside thinner, I've never done it, but it's what I plan on doing for the two that have the warped covers. I think that would work.