I have applied 3 coats of poly so far. I'd like to smooth out the surface before the final coat. What type of sander should I use: Square Buff Floor Sander ,
Random Orbit Floor Sander ,
or just a hand held orbital ?
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Depends on the quality of the existing finish... if it's as flat as you want it to be, then I'd kiss it with 150 on a pole sander.
If you need to knock down blobs/ runs/ etc, then start with 120 on the handheld random orbit sander. Work your way up to 150/220.
Vacuum and then wipe with a compatible tack cloth to remove dust and residue.
If you're having a hard time laying down a perfect coat, make the last one a bit thin -- 10-20% added paint thinner -- and it'll flow better. Also, don't skimp on the brush quality.