Have a question about removal of 2 year old solvent based sealer that the customer was told to thin with xylene. This is about a 20k sq ft job but their is only a small section on the back patio that has this stuff on it. They stopped because of what it was doing. It has a nasty yellow tinged to it and you can see exactly where they put it on. I'm sure pressure alone will not get it off without leaving some nasty marks on the patio. I didn't find anything in the search but one post and it was questioning oxalic acid. Has anyone sprayed water and then some acid on top and let it dwell for a few minutes? I used to acid etch concrete before painting shop floors but that was bare concrete. Looking for suggestions folks.