Artie66,
Yes, the tank is aluminum, what has happened is that water in the fuel, being denser than the petrol, settles at the lowest point in the tank. Over time it oxidises the alloy, turning it to a powder like state in molecular sized corrosion leading to a tea bag effect where the eater settles.
I've just repaired mine. You only need to clear the area affected where the paint is bubbling, back to the alloy, then using a fie tip on a Dremmel, carefully clean out the corroded holes. This led to me having several holes, no larger than 3mm diameter to fill. I used a 2 part plastic padding fuel tank repair sealant. Goes off after 5 mins and you can sand it down easily. After coats of primer AMD filling inbetween coats, just used a spray can and laquer to finish.
USE CARE WITH THE TANK ! DRAIN IT AND LEAVE IT A COUPLE OF DAYS TO EVAPORATE ANY FUEL, THEN I FILLED THE TANK WITH CO2 FROM A FIRE EXTINGUISHER BEFORE CLOSING THE FUEL CAP.