Got all the components off a frame recently, scraping the paint away any areas there is rust underneath before spot painting. Though I noticed again there are rust particles in the seat stays that are audible anytime the frame is rotated. Although not a mechanical issue per se, it seems worth looking into how to remove the excess material as part of the restoration. Suppose there is a rust solvent that can be injected into the drain holes that would dissolve the rust?

Flood the area in Pb blaster and slosh it around. It should break it down releasing the now smaller particles out the limited size drainage ports.

Evapo-Rust is a chemical solution that addresses rust quite well. If the frame has vent holes one can soak it in a bath then drain. Frame Saver and other copies are a rust preventative coating that can be sprayed into the frame as well. Andy

Really? PB breaking down rust? More like it offers a thin solution to flush out loose rust with. Not a wrong choice but not one designed to attack the on tube rust. Andy

Using PBB for projects I often get involved with [similar as the OP is facing] has shown great results for attacking corrosion; breaking it down small enough to allow it to pass thru tight areas. Not as effective with aluminum tho.