How can I remove old wallpaper paste that has been painted over?
Answers:
Sorry, you scrape it off and then patch your walls.
Other answers:
thinner
thinner
If it's too hard, the only option may be sanding.
Sorry, scraping / sanding is your best (only) option.
Rent a steamer from your local tools and accessories store. This will soak the paste and with a plastic trowel you can scrape of the residue. When dry take a mouse sander(palm held) and sand lightly.
Your best bet is to sand off as much paint as possible. Then soak the walls with water using a sponge. Let is soak in for a few minutes before re washing the walls with the sponge, make sure you keep rinsing the sponge as it gets sticky from the paste. It may take some time, but this should work. Be prepared for a long day of scrubbing the walls if it is big room and their is a lot of paste. Oh, the hotter the water, the better.
get a orbital sander and get a face mask your going to need it and to get a room to look good it allways takes elbow grease