i have a similar problem on a vintage frame in an impossible to match color. the rust was plenty, but small in diameter. i started by polishing with a buffing wheel. that removed the rust, but uncovered those black veins. i bought a glass fiber pen to get to the more stubborn spots and worked down to bare metal. at first i wanted to go the touch up route, but could not match the color for the life of me. regardless, in spots where i didn't even see surface rust, i had pitting below the paint.

i asked myself the same questions as the OT. in the end i decided to strip and repaint. i still agonize over the possibilty that MAYBE i could have gotten the bike built up quicker and with a lot less effort, but hey I want to ride in berlin germany and in the winter. rust can't be a problem. who knows if i will sell the bike or keep it forever, but a good repainted vintage frame can also fetch some serious coin. a serious buyer will always look at a rusty frame as just that. a frame which needs a lot of maintenance and or prep work to be restored. just my 2 cents.