Cleaning up waterproofing membrane

I just finished a full gut/remodel of my master bath. For the shower I put in an acrylic base:

https://www.lowes.com/pd/DreamLine-S...-Drain/4573408

For the tile surround I applied Mapei Mapelastic AquaDefense to the Durock before setting the tile.

https://www.lowes.com/pd/MAPEI-Mapel...mbrane/3056573

I did an OK job cleaning up after myself, but now that I've cleaned out all the grout and mortar and dust and everything from the shower pan, there's a lot of remnant of the Mapei in the grooves in the shower pan.

Does anybody have any advice or success story in cleaning up this stuff after it's dried? It's pretty stubborn stuff, and I don't want to scratch up the acrylic pan too much. Naturally, since it has a bright blue color, it's very noticeable even when there's just a little bit still there.

 

I would try muriatic (hydrochloric) acid. It is often used to remove Portland cement based mortar. Basically it dissolves the Portland cement that holds everything together.

Muriatic is the strongest acid you can easily buy so handle it with caution. Actually read the instructions and use the recommended safety gear. Also, test it on a small area of your shower pan to make sure it doesn't attack the plastic.