[morzh]

I believe, an instruction simply said using water-saturated rag to wash the jelly off.

The jelly is applied to well-sanded area. Then after application and removal you look for specks of red (rust), and id it is still there, repeat the process.
The jelly, after the excess of it is washed off, will form some sort of compound with the residual rust/metal, that is stable and could be painted.