any exposed metal will flash rust.
What concerns me is that you have no liner for protection in there...why is it exposed metal to begin with?
I would look into POR15 products. The tank looks clean other than the flash rust, so if it were me, I would use the POR motorcycle kit to seal it:
POR-15 Motorcycle Fuel Tank Repair Kit
POR-15® Cleaner Degreaser to remove gum, sludge, varnish
POR-15® Metal Prep to remove rust & prepare tank for sealer
POR-15® Fuel Tank Sealer creates a permanently sealed tank
I did mine and am extremely happy with the outcome.