Soldering to steel (fuel tank)

Silver solder would probably be my first choice

OK, will try!
15%?
i

Damn that was fast !!!
I'm not an expert on brazing, welding etc but probably 15% would be fine
--others might chime in ...

Try straight hydrochloric acid, in standard muriatic dilution. That's my last resort for soldering steel.
Yes. And even silver braze can be tricky with brass. You have to be *very* careful, or the brass goes "slump...."
It depends. Common brass/bronze brazing rod can be straight yellow brass, although most of them contain other stuff. You want an alloy made for brazing brass, like Silvaloy, which typically contain some silver. Don't overheat or you'll be sorry (I have, for example, some very curious-looking shop-made yellow brass thumbscrews, with brass heads on steel screws. They're all wobbly from silver-brazing in a hurry and overheating them.)
But you should be able to solder that steel. Have you tried scratch-fluzing it with a throwaway stainless or plain-steel brush? It works. Then throw away the brush.

Personally, I think he's probably not getting the steel hot enough because all the water in the tank is sucking away the heat.

Or he's overheating it and oxidizing the surface before the solder gets a chance to stick.
There's three things you need to make sure of before you try to solder something (or braze, for that matter).
It needs to be clean.
And it needs to be clean.
And, when you get that done, it needs to be clean.
Making sure it's clean doesn't hurt, either.

OK, I am done. I used a steel fitting on steel tank, and brazed that stuff with brass brazing rod.
The result is not pretty, but completely functional.
i

Pics, or it didn't happen!

Bravely done....
Or, at the least that's what appears to have been printed on the underside of this here bottlecap.
Cheers...

Easy as - get the right tool for the job, in this case a very large plumbers soldering iron - the type that was at least 100 watts, with a 1 inch diameter bit in it that was the massive heat reservoir needed. (There was even a pump up gas fired one for the real heavy duty stuff.....) Gas axes are iffy, unless your pretty good at brazing - I aint, but soldering works fine long term on the very few that I have done.
Andrew VK3BFA.

I'd ask a couple of questions: -What was there before you started this process? If there was any other kind of material, some remnants could be fused into the area you are trying to tin. especially epoxies.
-What kind of solder are you trying currently? If it is soft solder, does it have a flux core, and, if so what kind? (Acid or Resin)? or ? If you using "electronic" solder, it may be a resin (rosin core) flux. I'd certainly go with acid core for this application. I usually keep liquid or semi liquid fluxes of both type around.
-Can you get tinning to occur on other pieces of sheet metal?
1. The area to be "tinned" needs to be bright and shiny, and it should be cleaned immediately before attempting to heat/tin it.
2. Use a soft flame. It is very easy to overheat something as thin as a fuel tank. One instant of too much heat and it's all over, until you re-prepare the surface.
3. If you go with a silver solder, make sure you get a flux that matches the temp range of the solder.
4. Years ago, we had to soft solder electrical wires onto steel leads from high pressure liquid sensors. The only way we could get tinning to occur was to use sal-ammoniac as a flux(sp?).
I personally still like to use heavy soldering coppers (soldering irons to the rest of you) for such work. I heat them with a propane torch until the solder to be used melts freely, then use the heat from them, (properly tinned, of course) to transfer the heat to the work. You'd be surprised at how effective this approach is. This way, there is NO chance of overheating the work. I like to collect old soldering "irons" for just this use. Note that, in their day, they were usually sold in pairs. One would be being reheated while the other one was in use.
Let us know what eventually works out for you. Pete Stanaitis ---------------

If you didn't have the steel fitting, you could have used the brass and avoided melting it by brazing a coating on the steel and then soft soldering the brass fitting to the coating.
Pretty is nice, but functional is much better.
Bob

For steel, you need acid flux, either the paste stuff or liquid, see the hardware store. With that much surface area, you're going to need a LOT of heat, unless you've a got really massive electric iron, a torch head with a big soldering tip will be needed. For that sort of work, I favor the old soldering coppers, those were designed for sheetmetal work. Used a kerosene or gasoline blowtorch to heat them up, propane works if you've got to be more modern. Applying a torch flame to solder, even with flux, will give the results you describe, just rolls up in little balls and doesn't stick. Tin the area first, then the fitting, then sweat the two together. You have to have more heat going into the area than flows out. Get the flux residue off afterwards, it'll rust the steel. Baking soda solution should neutralize things.
If you're going to try brazing, use a filler with a high silver content, it'll flow better and not take so much heat to melt it. If you've got ANY lead-based solder stuck on the parts, you need to get it off first.
Stan

You've already accomplished what you wanted to do, but I'll mention that I've experienced this before too. What has worked on one or two occasions was to scratch the solder into the steel with a screwdriver, coarse steel wool, sandpaper or the tip of the soldering iron while all was still fluid. That made the solder stick to the steel, "tinning" the steel first. The flux that I use is made by Kester and is a pink liquid. I bought it at a yard sale years ago. I have no idea if it's still available.
RWL

A steel or stainless brush is great for this. It also solves a lot of problems with soldering unknown aluminum alloys.

You need good flux to solder the steel. It needs to be CLEAN> Brazing works a bit better, but it still needs to be CLEAN. There are quite a few different kinds of "brazing rod" - some of which melt hotter than others. Same with solders.
And you need to get the METAL hot enough to melt the solder (or braze) - which requires a LOT of heat if the tank is full of water - which is why I use CO2.

It's called, 'Ruby Fluid', and was sold for copper plumbing:

kester has dropped all pluming solders & fluxes from their website, but that was a common product. It's harder to find these days, since they switched to lead free solder on water lines, but some stores still sell small botles of it for under $5.

$5.81 for a pint.

Did you do a pressure test?
I've had several gas tank braze jobs that looked beautiful but leaked right through the brazed area. A bit of air pressure, some soapy water and watch for bubbles...