I power washed a flagstone patio encased in concrete. Looks alot better... but there is still some tar/acid/tree juice spots all over it... The patio is under a very large pin oak... I can blast this stuff off a little with the power washer, but it would take foooorever. I have to be about an inch above the patio to wash this stuff off... Is there a bio degradable / non-toxic patio cleaner stuff that I could use to remove this stuff?