Lets say I have a bow in the plywood. I put tape on it and glue it to the wasteboard with weights holding it down, it’s going to flex, then get surfaced and when I take it off it will flex back out of shape.
I’m curious the best way to go about this.
Should I only take and glue areas which aren’t high points and not use weights to hold it down?
I would recommend 2 way tape, use clamps and be careful when its time to separate them
I would recommend another piece of plywood. Surfacing plywood will likely expose the filler layers which often have knots and voids. Plywood can bow even when flat because of heat from the cutting process. When storing plywood make sure it is flat against something and not proped up against something. That introduces a bow. Storing plywood sheets is problematic because of the size and space in almost everyone’s shop. If you must use plywood then the super glue and painters tape would work best. @1Eric suggested double sided tape but that really fouls up the bits. Painters tape can leave residue on a bit but for me it is easier to clean then double sided tape. You dont need to cover the entire sheet of plywood with painters tape only across the middle. Be sure to use a J roller to secure the painters tape on both sides before applying the super glue and put a weight on the middle until the glue cures.
As Guy mentions, you only have so much to work with on that first ‘ply’ (layer). How much is the bow?
When I surface a bowed/warped/twisted piece of hardwood, I want to leave it in it’s relaxed state rather than forcing it to be flatter. I usually set the piece cup side down in position on the machine & mark the outline. Move the piece then place several squares of painters tape on the spoilboard. Place a blob of bondo on each of the squares then set the piece back in place and push it down without deforming it until the bondo hardens. Now you can surface the top side, flip it & surface the back side & get a pretty flat workpiece without it springing back.
This will only work with your plywood if the bow is small enough that you won’t be cutting through the outer ply.
Thanks appreciate you guys offering advice.
It’s probably 2-4mm from the flat point to the bowed point.
I think I messed up by putting the full plywood sheet on a workhorse to use as a makeshift work table for 30 minutes. It used to be very flat.
I need the sheet to act as the base for a drum sander so it needs to be very flat.
Can’t you just turn it over so that gravity will the cause it to be flat?
I probably wouldn’t use a plywood, IMHO it might still warp after you screw it down. MDF or particle board stands a better chance.
If using plywood for a surface that needs to remain very flat I would recommend a 3/4" or thicker Baltic Birch plywood. It is considerably more expensive however it usually contains more laminated consistently thick filler layers of much better quality (can be machined down to thickness without the fear of knots and voids that occur in standard plywoods. It is considerably stronger and for more apt to stay flat.
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