Uncupping a cupped top

I posted recently about how the top of the “Spider Table” I made 15+ years ago had developed a bad cup from the sun hitting the top surface and bleaching it out.  This caused the z-clips to pop out so the table was loose, and it was rocking on the base.  Not great.

The top of the table is badly cupped, there is at least 1/8" of light under the straightedge...

The top of the table is badly cupped, there is at least 1/8″ of light under the straightedge…

I sanded the top to remove any traces of the old finish (and stains and deep gouges), and led it face down on the garage floor for a week.  I misted it with water on both the top and the bottom once or twice during the week.

Tabletop, cupped side down on the floor.  I moved it off the MDF and directly onto the concrete after this picture was taken (and after I cleaned up the mess in the shop).

Tabletop, cupped side down on the floor. I moved it off the MDF and directly onto the concrete after this picture was taken (and after I cleaned up the mess in the shop).

Yesterday I checked it, and guess what?  It’s flat (well, flat-ish).  The cup is completely gone, although there are some small waves in the surface.  But it’s hugely better, the pictures don’t do the improvement justice.  I can do a bit more sanding today to smooth out the surface and get rid of the coarse sanding scratches, then layer on more finish.  I start with linseed oil, and probably spray a shellac topcoat next weekend.

Look Ma, no more cupping!

Look Ma, no more cupping!

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5 thoughts on “Uncupping a cupped top

  1. Carl

    He I Joe – I strongly suspect that the cup will return. Biasing the moisture content is temporary and the wood will do what it wants when it reacclimates. Same thing happens to me when I glue up bass necks with Titebond. Introduced moisture causes movement which eventually rebalances.

    Perhaps cutting relief grooves into the underside will help disrupt the continuity and traction of the growth rings?

    • You might be right Carl, especially if it’s just current moisture levels that are causing the cupping. My hope is that uneven drying rates between the faces is the cause (and cure) for the cupping. I’ve left the now-flattened top alone to rest in stickers for a couple of days, we’ll see where it ends up.

      • The sunlight is the most probable cause as you say. Milling a few grooves in the direction of the grain is likely the most effective way of reducing tangential movement if it is seasonal moisture.

        Isn’t wood “fun” when it decides on its own course of action?

  2. Here in Rhode Island to get cups out I throw it on the lawn. I’ve never had one board that was cupped, completely flatten out doing this. Your garage did a good job of getting most of it out.

  3. Nice! I’ll have to file that trick away for later.

    Greg

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