Maybe I should say--easiest, least messy...I've already tried sanding, which worked but was very time-consuming and dirty. It's in my kitchen so it takes almost as long to put plastic over everything. Lately, I've been using a small 1" putty knife, which surprisingly worked pretty well on the thicker stuff. Some of it lifted off quite easily, but I think I've hit an impasse now. Is there anything that could be applied that would "dissolve" the rest of it? I plan to use Gardz when I'm done, but the wall is not smooth enough yet. Would skim-coating work better? I've dinged the wall in a few placed with the knife, nothing major but I will have to patch up some areas. This wall is about 8x8 and the mastic is not over the whole thing, it looks a little bit like someone put Elmer's glue squiggles over it.

Thanks for any help!