How many times does this occur? They are yet to sand right? You don't judge a floor from 3" away either.

Its frequent. They did 2 coats of finish already and will be sanding (180 grit) and doing third coat of finish tomorrow. obviously we dont judge from 3 inches away, but for brand new floors we want them to be perfect. in any event, we are just curious as to what exactly it is in between the boards?

You are dealing with dry humidity causing gapping. You can have installers fill the gaps prior to finalizing with a final coat. Make sure they clean off all filler residue. Otherwise, you will have blotching from fillers. I would buff with 220 grit and not 180 grit. Once that heat goes on, you will have larger gaps.

The wihte stuff maybe dust from the inter-coat abrasion. Which is not needed between first and second coat of Wood Look. Contractor just needs to do a better job at removing dust.

Thank you so much - appreciate all this info! will relay!