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I have a set of Amazon Rosewood I am working with. I joined the back plates and then stopped working while we moved. That was almost 9 months ago. I still need to thickness the plate. So now I am looking at the wood and the lower left section of the back plate has a cup in it. The rest is flat. I have read that you can wet one side and let it dry with weights on it, but I can’t find info on which side to wet. Inside the cup or outside?

John

Inside the cup. The side that is losing moisture shrinks, causing it to cup.

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Aoibeann

I wouldn't add water just set the cup down and let the wood aclimate.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I wouldn't resort to water just yet. If it is already acclimatized to the shop, I would thin the plate first. If I get cupping after joining I am planing it selectively, if possible: insist on the center on the convex face and insist on the edges on the concave face. When down to thickness you need to judge on how much force you need to use to flatten it or press it in the bracing dish. Slight cupping that doesn't show a lot of resistance is not a problem I think.

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Build log

If it is just a little cupping on the corner - I would try a clothes iron - try it dry first with a layer of cotton in between. If that does not work then try to mist the cotton a little bit. Should be just like bending a side on a hot pipe. Just don't burn the wood.
for large cupping I have tried putting it on the concrete floor of my garage and then stickering -

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It's this new idea from recent decades that everyone gets a participation award. - MUX

I never resort to adding water . You want to get it out. I agree that after thickening , if the cupping can be controlled with minimal force just work with it. I would put the dry side in as moisture from the glue can make things move a bit. Adding water just adds to the problem.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Thanks for the things to think about. I'll have to give some things a try over the weekend. I'll wait for the water until I need it. I should be able to pull this one off. Then on to the next question.