Step 1: lead loading and safety
The process of loading lead is an old way of filling dents and smoothing joints in metal work before painting it is also waterproof unlike plastic fillers.
It can be used on most steel surfaces but remember safety first you wil be using heat so make sure there is no flammable stuff about, fuel tanks must be washed and well ventilated, lead is also not good for you so do not machine sand or handle with out suitable protection. You are responsible for your own safety and I am only guiding you on how to do the technique not how to make sure you stay out of hospital or worse.
First step is preperation this tank was aqua blasted to wash out old fuel and remove rust, it was then coated with flux and tinned.