I noticed the chain stays have some rust on it too, so I taped the tiny holes close to the rear axle and injected evapo rust using a syringe on both chain and seat stays.
Took the ride and did as well for the front wheel forks.
Seat stays are pretty small (well my frame is small, 49cm) so I have filled with some 40 ml each, The chain stay got some 60 ml each and the fork was like 50 ml.
Now, thinking on how I will send some hot air through the tubes to dry it out nice. I have the heat gun, so have to redirect the air flow to a tiny hole....
Seat tube has some rust towards the bottom - I can see easily. I will check the down tube with some fiber optic camera I got some time ago.
Depending on the rust amount, I will fill up with evapo rust as well.
Maybe seal both sides of the head tube and fill in both top and down tube at same time.