Newby here. I am installing 400 sqft of 18 inch high quality porcelain tile on a concrete slab. I removed the vat and wet scraped off the cutback but there are some spots where water was beading up because I believe that also tar was used to seal existing cracks and fill low spots. The cracks are 1/16 inch or less and very close to even vertically. I did scrape the tar down to the surface except on the cracks where this isn't always easy because of an uneven concrete patch on a cut out repair they did next to the sewage sump pump and around cracks.
So the issue is getting a good bond. I was initially poking some holes with a cold chisel, but this is getting pretty tiring and messy. I have also worked on some of the remaining residue with water cleanup 3m paint remover.
So at this point I am hoping that the subfloor is ready, and after doing some test tiles on thinset, i will use self leveling compound for the low spots, but I am also concerned about getting a sufficient bond.
So here are my options in terms of better bond strength
o rent scarifying machine (big mess, long drive)
o drive my fracture guard 5000/versabond 100 miles back and swap it for red guard/flexbond
o take back fracture guard 5000/versabond and use instead ditra/unmodified thinset