One thing that you MUST do if you are building a finish out of poly is sand between
The reason is not to keep the surface smooth... but to allow mechanical bonding between the coats.
Polyurethane is a long-chain polymer that cures without leaving any open bonds available for the next coat to adhere to.
Soooo... if you apply another coat on top of an un-sanded poly finish... the newest coat will be 'floating' on top and not bonded well at all the the lower surface.
Sanding between coats of poly is necessary to allow for mechanical bonding... essentially you are roughing up the lower surface enough to allow the new coat to find some 'broken' polymer ends to adhere to.
320 grit or finer sandpaper is sufficient.
Uhhhm... wood professor rant mode off!
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