Yes Andy i realise that is is not wet, But may of been in the past. You can seal edges with pva glue but this will prob expand the chip board before is sets, then you could sand and then seal again and so on but chip board has no guts to it and bits will come away again and again! I would get a plywood box , i thought you got one in the deal? For all the work trying to fix a unfixable box,you are better off with another box made of ply, I will bring my box of
matches when i come over, let it go son!!