I couldn't really notice any low points, but I'm thinking a finer grade paper to smooth it out.
Makes sense with the 'gap'- i'll post back with my final OCD sanding.
Not recommended. A slightly 'rough' surface finish, such as you have, will grip the diaphragm edges a lot better than a polished finish. Which is why I specified those abrasive grades.
Now check the carby surface, as AVB has said.
You can use the cover plate as a 'flatness reference' now.
Smear the sealing surface lightly but evenly with say dark coloured grease, 'bearing blue' as used by fitters, or even lipstick, and press it against the carby surface. Then remove it, and look for 'windows' of non-contact, where the coloured paste has not been transferred to the carby sealing surface. If there are any, it isn't flat enough, and can be lapped in the same way.
