I've soldered fuel and kerosene tanks as well. Scuff the area with the hole(s) down to bare metal and get it fairly rough. Then apply acid flux, get it good and hot (one of those big soldering coppers used for copper gutters works well), and apply solder. If it didn't tin, you didn't get the steel clean enough. If you're trying to cover a large area of pinholes, you probably want a piece of tin can applied to the outside of the solder patch. Prep it the same way, get it and the solder spot hot, then press it together to make a solder sandwich. Once the solder is liquid capillary action will do the rest for you.
The whole thing can be done without any open flame nearby, although I always seem to end up using a plumber's torch to heat up the soldering coppers - just do that upwind.