I solder holes in tanks before sealing them, well before cleaning them before sealing.
If I am using the big iron I use plumbers solder, but last tank I did I used a 45W
electric iron and electrical solder. I use phosphoric acid as flux.
I think that is the top, condensation, will be very rusty inside, very very rusty, I would seal the inside after soldering, it will need a silly amount of cleaning internally first, looks a bit toasted to me,
if its been blasted the rest of the tank will be sound .warm the tank up dip the solder in flux and dab over the holes then the flux wont burn off .just heat sufficient to melt the solderBlow torch sealed a few of the holes but as there's lots to do it's hard to regulate the temperature and the flux is overheating. Will try the soldering iron tomorrow.
Does a 45w iron work? Because that's all I have.
The tank is otherwise very good. It looks like the paint just failed in spots. When I was blasting the tank I could see that 99% of the paint was good aside from those pin holes.