I do it after finishing, I take the control horns off external servos and cover them with blutac to stop gumming up the motor with paint. Pull the motor out in the power pod and stuff all the apertures with newspaper.
If it’s a brown FT board build then key all the surfaces with 600 grit sand paper or a medium Scotchbrite pad before assembly, whilst they are on the frames. Seal the exposed foam edges with PVA or iron them round to seal the foam in case the aerosol solvent melts it. Lots of light coats, prime with a mist coat.