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

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.


The curing process starts after it is applied. There are degrees of
'cured'.

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.


24 hrs is plenty of time to lightly sand with 320 wet dry.

I've noticed that before the recoat time, I can sand lightly, and only
to remove raised grain.


What do you think I am talking about here? Re-read what I wrote above.

The poly is still soft enough that if I try
to sand into it, it just rolls up into lumps (like a pencil eraser).


When it behaves like this during sanding your poly has not 'sufficiently'
cured. Granted curing time is governed by humidity and temperature.

Only after it's fully cured is it hard enough to actually sand into
the poly.


Wrong. It does NOT need to be "fully cured", only sufficiently cured.

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


I get the same results when 'sufficiently' cured. Doesn't have to be "fully
cured".


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.


Sanding is usually required after 24 hrs.