One comment, don't sandblast with sand, use a less aggressive media. Sand will
or can remove a noticeable amount of metal if used by ignorati and a tattooed
frame is not desirable. Your frame probably has enough thickness in the metal
not to be weakened significantly but some frames are thin enough (submillimeter
thickness) that sharp sand can be used to cut the frame in half. Your masking
material has to withstand the powder coat heat treatment, so inquire of the
powder coater what to use. Cover all bearing and threaded surfaces. Dropouts
are not bearing surfaces. Seat tube probably is OTOH, as clearances here are
pretty snug and cleaning out the seat tube would be a PITA.
sch is right: you want "bead blasting" not "sand blasting". One shot of sand and your bike will look like Swiss cheese!
Any decent powder coat shop will take care of plugging the tubes before powder coating. Are you having a fork powder coated? If so, you'll want them to mask the area where the crown race seats. Also plug anything with threads (e.g. disc brake mounts, water bottle bosses, etc). Finally, they should mask areas where the axles seat. My shop did this, claiming that the pressure from clamping the QR skewers would eventually cause the powder to crack. Sounds like a good idea to me!