Do I need to grind down my concrete slab floor for a new bathroom before I apply membrane (HydroBan) before tiling? The slab is already quite flat, just some join compound/paint splatters.
The short answer is no you do not. Clean up your joint compound if it is a bump above the surface.
HydroBan usage around fixtures: Bonds directly to metal, PVC and ABS plumbing fixtures only.
How to use : this link does not seem to work in Firefox 57 but will work in MS Edge.
How to use HydroBan video : watch it and you will see them doing exactly what you are asking about with no grinding or roughing up the surface.