G'day Allan,
I personally wouldn't powder coat a petrol tank, the silver is a bit dull & very hard to touch-up if it chips.
Not trying to be smart, but silver is pretty much silver, & the only variation is the size of the sparkle.
I would mask up your side panels that remain chrome, rub the remainder of the tank down with 1200 w&dry,
then etch prime followed by silver base coat, then multiple coats of 2-pack clear. I always use the fine sparkle
on older bikes, back in the old days they would have probably used silver enamel.
regards, Cus