MIMF

Just bought a new back and when it came it has like a 1/4" cup in the middle of the board.
Should I send it back or try to straighten it? If straighten it whats the best way to do it?

thanks

If the cup is a result of flat sawn lumber, the bracing will fix it with no problem. If its with the grain, I'd be calling the supplier.

John, what is this a back for?? What wood is it?? Where did you get it from?? How long has it been in your shop?? What are your shop humidity conditions??

Was it laying on top of the bench or left leaning against a concrete exterior wall?

Back is for an OM guitar. It is quartersawn walnut.
Came cupped in the mail on Friday from A&M WoodInc.
My shop is at 40-45% humidity.

If it were me, I think I'd let it sit upright, or stickered, in the shop for several days so it can equalize on both faces and settle down to what it inherently wants to do before getting too excited. Who knows how it was stored, humidity where it came from, etc., before you got it. Then see if you've a problem or not. For gosh sakes don't try to "fix" anything right now.

Chuck, aren't you supposed to be on the beach gathering data on bikini trends?

I'm back.

Board still hasn't flattened. Company suggested spraying the underside of the cup with some water. Tried twice but it didn't do anything.
Any other suggestions?

1/4" is nothing; just glue it up and once thicknessed and braced, it'll be fine.

I would try laying a damp dish towel on each side of the thing and gently ironing the convex side it with a hot clothes iron.

You want the wood dry, adding any moisture is not good! It expands the surface as you might think and will induce a dome but when it dries it actually makes things even worse because the wood will try to return to its "normal" state and shrink which will further act to take away the dome.

The trick I use with old warped bass bridges is to boil them and completely saturate them until they return to their "primal" state. Then I hang them and let them dry for a few weeks.
Ironing just speeds up that process, steam penetrates and then dries almost instantly provide you keep it hot enough.

Either this wood wants to be cupped or it doesn't. Best to find that out before you start building with it.