Dented fuel tanks come with the territory of motorcycles. Either a motorcycle is a well-used project of it's simply become battered because we're clumsy allowing something to fall on our nicely painted fuel tank, or it can be damaged by an accident. Badly mangled fuel tanks can be resurrected with dent pullers, but you'll still have imperfections that will need addressing. Smaller dents can be easily sorted in the average shed or workshop.
Modern two-part body fillers are the choice of industry because they're cheap, easy to use and most importantly, don't require significant staff training.
However, before the invention of body fillers, lead pudding was the only viable route to sorting an industry standard, the technique is not only still viable, it's also extremely effective.
The bond created between the steel and the body solder is at a molecular level. This means some solder penetrates the steel and some steel migrates into the solder forming an intermetallic compound forever attached to both materials. Vibration can and will disrupt the adhesive and cohesive bonds between steel and body filler, but a lead-puddled repair is effectively permanent. Although the technique might appear daunting at first, in reality it's not as hard as it looks. With a little practice, surprisingly good results can be easily achieved and it doesn't take long, either.
Fuel tanks are remarkably awkward things to hold in place. By their very nature they're only happy sitting in one position. Unless the dent is perfectly horizontal the fuel tank will need to be somehow positioned to facilitate an effective repair. Blocks of wood, hessian sacks or similar make can normally be configured sufficiently to keep the target area where it's needed.
When it comes to dressing back the excess solder, a solid surface is obviously required so if blocks of wood don't work an alternative is needed. A second pair of hands holding the fuel tank at the correct angle makes the job infinitely easier in our workshop.
Screwing blocks of wood to the bench and allowing the fuel tank to butt against them can also work if you're on your own. It's also viable to clamp the fuel tank down with a ratchet strap, but just make sure you don't apply too much force or you'll have another dent in the fuel tank to fix!
The body solder you need to be using must be either 20/80 or 30/70 tin/lead for one very simple reason... any other combination simply won't allow you to puddle/fashion/mold/manoeuvre the blob of metal you've just deposited. Sticking to these ratios gives you an alloy that has a large amount of plasticity and this, in turn, allows you to shape what amounts to a semi-molten metal. Use electrical solder and chances are it'll run out of the dent like water. Tallow works well as a puddling aid because it doesn't char easily, plus it's possible to wipe it off and apply more solder paint and/or solder as needed.
Try doing the same with engineering grease and it'll all start going horribly wrong.
The chemicals in solder paint are quite aggressive, so it pays to either work outside or ensure there's good ventilation. Although the process utilitezed lead the chances of lead poisoning are extremely remote. Short of consuming the stuff you'll be perfectly safe.
If you find a need to dress the puddled area with a mechanized sander we'd recommend working on damp cloths to dampen down any dust, and wearing a suitable dust mask. Residues from the flux in the solder paint should be removed with water to prevent corrosion and any remaining tallow can be evicted with a rag dampened with brake cleaner.
Finally, never work on a fuel tank that you suspect has fuel residue. Obvious, but some folk have done just that.