I have been doing sheetmetal
I have been doing sheetmetal work on airplanes for a long time and 90% of my work is changing corroded parts on aircraft. I can promise you that no amount of paint will protect your frame from rotting in half in a few years. The amount of measures that aircraft operators go through to prevent corrosion far exceeds painting the structure and it doesn't work. You would have to pay me to take an aluminum truck, airports dont use salt on the runway like the state roads do in most states so if you live in the north anywhere then you might get 1.5 to 2 years out of the truck until you walk out of your house and find a pile of dust where your bed used to be. If you do buy a truck get a corrosion prevention package if you can so you can get a new truck every couple of years. All it takes is one piece of gravel to fly up and chip the paint and you will never notice until your foot goes through the bed or you take your bed off the truck once a month and go over it with a 10x.