Support Painting Doesn't Work on Top Surfaces

I have a model that will require supports to root from a few top surfaces. I’m trying to control which top surfaces can be used as a base for the supports, but support painting has no effect. I can paint top surfaces just fine, but it doesn’t do anything. Is this a big or a missing feature?

Have you selected “build plate only”?

There are a few specific locations where supports off the build plate won’t work. Also, there are a few top surfaces that will end up covered. I’d like to use those areas as a base for the supports.

I get that. I was wondering if you had it checked without realizing it.