Hi AVB,
You'd still require a copper bath prior to brass.
Zinc would probably be the best and cheapest method but you'd never be able to plate internally as it's a "Low Current Density Area" unless you can get an anode to sit inside the tank and not touch the tank itself. This is the real art of plating which I was involved with. Definitely not your average back yard process that's for sure.
Generally most fuel tanks are Galvanised as this method gives coverage both inside and out. It can be a costly process as a one off job (around $60) unless you know someone that deals with a galvanising plant on a regular basis and you can put your gear in with their mass amount and this way it won't cost anything.
Most galvanising operations just don't want to deal with the average wood-duck and one off jobs, they just want to do larger volumes and do it as a mass production line situation.
Cheers,
BB.
