"sherwindu" wrote
Is there a better way to clean off this river junk?

I use an oxalic acid product called Savogran Wood Bleach from Lowe's. I mix
a tablespoon or so of the crystals with water in a quart spray bottle and
mist it on the stains. A few minutes later they're gone and I rinse with
plain water. No ill effects so far. I have noticed that a recently waxed
surface will make the OA mixture bead up and be less effective. YMMV