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DJ Delorie DJ Delorie is offline
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Default Should I use mineral spirits after sanding between polyurethane coats


"Stoutman" .@. writes:
Once cured there is nothing wrong with wiping down with mineral
spirits. I have done this with as little as 8 hours curing time.


Cure time depends on the poly. The stuff I use isn't cured until at
least a week passes. After 8 hours it's only dry, but not cured.

And I hope by "finally dried" you mean "I waited as long as the can
said to wait". If you don't recoat in the time specified, you need to
wait at least a week or two (to fully cure) and sand before recoating.


B.S. You can sand after 24hrs. I usually use 320 grit wet/dry
paper and lightly sand.


Well, nobody is going to stop you if you want to sand whenever you
feel like it. I'm assuming the OP wants as good a finish as is
practical.

I've noticed that before the recoat time, I can sand lightly, and only
to remove raised grain. The poly is still soft enough that if I try
to sand into it, it just rolls up into lumps (like a pencil eraser).
Only after it's fully cured is it hard enough to actually sand into
the poly.

When fully cured, sanding poly should result in a fine, light, dusty
powder. Like what you usually see with sanding sealer.

So are you saying that you can only sand during the time specified
on can (3 hrs usually) and after one week? This makes no sense.


I'm saying that if you recoat outside of the recommended times, the
coat won't hold as well. Why? Because of how the poly layers join.
During the cure time, the poly molecules go from loose to tight, so if
you recoat during that time you can still hook into the uncured
molecules for a mechanical join at the molecular level. Once they're
cured too much, this is no longer an option for a reliable join, so
you wait until it's cured enough (i.e. hard) to sand it to make a
mechanical joint at the macro level.