Levelling off a surface for veneer

I've only ever done simple small veneers, but my research suggests that surface prep in terms of flatness is less important than cleanliness. All surfaces must be

• Acclimatized
• Dust free; really, really dust free
• Free of oils, including finger grease

Your results will only be as good as your prep. Just make sure the surface is reasonably true.

(As an aside, note that plywood is a type of veneer, so you are actually adhering two veneers together, if that helps with your research.)

If you have to use filler, you will have to do research to figure out if it needs additional glue preparation. This goes for any surface, really. But with plywood in the home shop, the assumption is that you will be using contact cement and a roller. That is, I am assuming you do not have a hydraulic or vacuum press.

So, your prep work will be more about preparing the surfaces for gluing up with a high-solids contact cement. This may include not just sanding to true (but not necessarily smooth -- check to see if your glue wants surfaces rough for good adherence) but also meticulous dry and damp dust removal, and probably at least one wipe with denatured alcohol.

You can also get adhesive backed veneers if you just want that convenience.

That's going to be a lot of filler and it would have to be a filler that bonds with your veneering glue well, also sanding it perfectly level is going to be a challenge. It's hard to advise without seeing the piece, but I would try figuring how to replace a warped part, or if that's impossible then whether a construction could be modified so that warped plywood part is level, i.e. maybe adding a support or a stiffener, gluing a corner block, gluing it to the butting piece, etc.

There's a chance that a veneer might straighten the warped part if the part is thin. On thin panels it is recommended to glue a backer veneer to prevent warping, so maybe this could be used to your advantage - gluing a thicker backer or not gluing a backer at all, depending which way you want it to counter warp. This approach gives no guarantees of course.

My plan is to apply wood filler then sand it down... but maybe there's a better way?

Typical "plastic wood" type filler is good for filling nails holes and small dings, but not always great for filling large, flat areas the way, say, auto body filler is. To fill a broad, 2mm deep depression, you'd probably be better off with something that's very strong and stable with good adhesive qualities, like epoxy thickened with wood flour or other fibers, or auto body filler.

So, at the point where the warp is at it's worst, the side piece sits slightly proud. I could sand down the side piece. Sounds like that would be a more robust option, but perhaps less aesthetically pleasing.

If it's not too late, I think you'd be better off removing the top of the case and replacing it with plywood that's flat. It'll never be as easy to fix your project as it is now, before you add the veneer. As jdv explained, the key to a good veneer job is a good substrate