Hi @HReno,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
What you are looking at is tile adhesive, not waterproofing, and to be completely honest with you, it is going to be difficult to remove.
Is the wall solid block or concrete? Or is it fibre cement sheeting or plasterboard?
I ask, because if it is fibre cement sheeting or plasterboard, you will save a huge amount of time by simply cutting out the sheet and replacing it.
If the wall is block or concrete, then you will likely have to use a combination of a few things, along with a lot of patience and persistence, to bring it back to a paintable standard.
When you say you have a hammer drill, what kind of chuck does it have? Is it a rotary hammer drill with an SDS+ chuck like this Ryobi 1500W SDS+ Rotary Hammer Drill RSDS1500-K? If you can advise what the model number of your hammer drill is, I can check what kind of chuck it has.
I ask, because the best thing you can use is a chisel bit, like this Kango 75mm x 170mm SDS Plus Angled Tiling Chisel or this Kango 125mm x 250mm SDS Plus Floor Scraping Chisel. The issue with this is that these type of chisel bits are only made for SDS and SDS+ chucks, so we'll have to confirm that you have the correct type of chuck on your drill. If you don't, then you could always hire this Small Breaker Drill, which has an SDS plus chuck, through our friends at Coates Hire.
Chipping with a chisel bit will remove the larger clumps, but then the thinner stuff will likely remain. To remove this, a wire wheel is likely the best way to go.
Things like some 80-grit sandpaper and a cheap chisel would also come in handy.
With any of this work, there will be some risks, so it is important that you wear personal protective equipment. I'd suggest you wear long protective clothing, gloves, ear, eye and hearing protection, as well as enclosed shoes.
Let me know what you think and if you have further questions or details to add, I will happily assist further.
Jacob
