The windings are coated with an insulated varnish. "Just what" depends on what wire was used to rebuild. It is possible that the solvents in whatever you use might dissolve that coating on the windings. Overspray on the slip rings would not be good.
I would disassemble the thing if you want to paint it. Not that difficult. Pull the brushes, pull the bolts, gently pry apart. You'll need a puller to remove the pulley, and will need to unsolder the diodes from the stator windings. You could mask the diode wiring / terminals from there
