Q&A for How to Remove Mastic

Yes, it doesn't have to be perfectly clean. You mostly want everything to be as flat as possible.

It can be a little touch and go with plywood, but it won't rip out large chunks of plywood, just the topmost layer here and there.

It would be faster and better to just remove and replace the drywall. That works every time.

Mortar requires an unsealed surface. If you throw some water down and it beads up, the mortar won't adhere to the sub-floor. Water should be able to wet the sub-floor. Even with all of the mastic removed, a concrete sub-floor may have to be ground down a little to get it wet. All that said, I had a concrete sub-floor sealed with an epoxy adhesive coating for sealing off water. I ground it down but never got it to wet properly. I used thin-set anyway and the tiles are still good after 5 years but there is almost no traffic.