The face ply of plywood is likely about 1mm thick.. I would not use a belt sander..
hand sand the surface of the entire sheet using 220G paper and a sanding block.. Sand the scratches as best you can. Depending on the type of plywood - I would use
Wood conditioner before using stain to prevent blotchy stain (pine will get very crappy looking if you do not condition the wood)
Then stain
Then Poly..
Put on lots of coats of poly to fill the grain. Once you have some buildup - sand between coats with 220G to knock off the lumps and bumps.
You can fill the scratch with poly and build it up..
You can also try a damp cloth and a iron (like you are ironing a shirt) over the scratch to see if you can swell it back to shape.. This works if the wood fibers are not cut/torn..
good luck.