After sanding everything with 80 grit paper, it's time for repairs. Most every cabinet I've ever done has one or more corners separating, along with a variety of other issues, like ply separating of the plywood.
The first pic shows typical plywood separation. You'll want to pry it open and blow any debris out with compressed air. Next, set the cabinet such that the glue you're about to apply will run down into the separation. While prying it open, apply glue. Once the glue has run down, push closed, then open again. This will distribute the glue throughout. You can also use compressed air to blow the glue into the joint but be careful. You don't need much air. I'll also you a razor blade to spread the glue in some cases. Continue applying glue until the joint is full. Now all you have to do is clamp. Once clamped, if you don't have glue squishing out, you didn't apply enough.
I don't mess with any excess glue until it's fully dried. Once dried, I'll remove with a paint scraper.
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