Removing Mastic From Drywall?

Ok, this is a follow up to a previous wood paneling removal question. The wood is off and the nasty mastic remains. I don't know why they used this but hey...it's my nightmare now. The drywall needs to be skip troweled once I'm finished but, for now...can I use a small electric sander to get that off or should I just go for a sanding block and is there anything I need to know to not tear up the wall any more than it is?