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« on: 05 October 2017, 18:56:35 »
not easy to tell from the pic but it doesn't look too bad. the main concern is the split along the seam, you will need to weld a plate over it. the blister (probably hole) looks minor, you could just clean up and fill with an epoxy filler.
so i would mechanically and chemically remove the rust to get a feel for what is solid and most of all to get a clean rust free surface to weld to. fertan chemical is probably the easiest to use as it is water based and doesn't dry, or an acid like Hydrochloric Acid (Muriatic Acid) while it is still legal.
i have an inverter mig welder but for that small section i would get out my stick/tig/plasma machine as it is less hassle to set up. you don't need a continuous neat weld, as a rough guide 1 inch of good weld holds a tonne. then grind it neat and treat the inside liberally with penetrating rust treatment and the outside with an anti rust primer, either water based like vactan or preferably solvent based like fortress, then underseal.