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I have two rough sawn planks of flat sawn oak that is kiln dried. They are about 2" thick and 25" wide. Unfortunately they cup a little more than 1/2"
Is there anyway to diminish the cup before I grab the plane?
I don't want to resaw and glue them up.
Not really. Resaw with a thin kerf blade & reglue & the joint should be pretty much invisible.
Wetting the concave side to swell that side till it straightens and then holding it flat while it dries may work. Generally better for softwoods than hardwoods though. Otherwise, rip and reglue or put up with a thinner plank. Don't forget that if you use them with the concave side down, you may get away with leaving a bit of a hollow to keep as much thickness as possible. Watch the end grain though. You want the exposed end to be straight. Cheers
Wetting concave side usually helps only temporarily ,if at all. Old tables with concave tops mostly got that way from water.
Top gets wiped off with damp cloth reapeatedly. That makes it swell and convex. That compresses the cells. Cells then dry out permanently narrower making the top concave. Google "compression ring set". Ripping and re gluing is probably a good way on this job, as water would be slower.
It seems you will have to reduce the thickness by 1/2" by planing both sides or to rip and glue once the new edges are jointed square with the show face. That can reduce the cupping but it may still show in each of the halves. I have just been working a bunch of rough cut lumber with cups and bows. Results in a lot of saw dust and a very messy shop. Good luck
Have you figured out why these boards cupped? I assume they weren't cupped when you acquired them. Were they stored laying on a solid surface. Was the humidity unusual in their storage place? Do you have time to store them well stickered in dry storage?