Hi Tom

The water proof sandpaper is not designed for the rough treatment of
sanding bare wood, but rather for the sanding of automotive finishes and
other more delicate final finish treatments IMO.

And yes the abrasives will be ripped off of the paper backing in
imbedded into the wood, and in light color wood you can get enough of
that to be visible, other people have also reported this.

However I do not see a need to use more expensive water proof paper for
sanding bare wood, there are better and less costly abrasives for that,
and when sanding a finish on wood the loss of abrasive particles should
not be a problem.

Have fun and take care
Leo Van Der Loo

Tom Storey wrote:

Is it just my imagination or is my 'Kingspore' brand waterproof sand paper
leaving black marks on some of my work? Ok, I know you can't see it!
However, I've been working with white birch and it sure looks like it's
darker in spots after I've sanded it, both dry and with a blo/thinner mix.

How does the 'dark red aluminium oxide' waterproof paper perform and will it
leave a stain?