I took a gas tank to a radiator shop a few years ago to get it cleaned (boiled out). When i got it home, i looked it over, put in one of those "sealer kits", put it back in the car, put a few gallons of gas in it and then watched all the gas drain out on the driveway. I got it back out, stood it up with the bottom facing the sun and looked down the filler to see maybe a hundred very small holes. I took it back, they cleaned it out again and soldered up all the holes and then it was ok. The process of cleaning it out opened up all those holes. You would be surprized. Same thing with body sheet metal, i lightly sand blasted lower quarters and rockers on that same car. They looked ok till they were clean but they were full of very small holes not seen before.
