There are no short cuts with plating. Any imperferction will show. If you skimp on the prep work you will be dissapointed with the end result. They will polish out the rough parts to remove imperfections then they (should) tripple plate, ie copper, nickle then chrome. Copper can be laid on thick and then polished to givethebest possible surfec forthe next layer. Polish again after the nickle, then chrome.
If the tank is rusty, you may want to look for a one in better condition to start with it will be cheaper in the long run.
If the tank is rusty any way, by the time the damage / rust is polished off the metal will be getting very thin....
Go to a good plater who will tell you all of this anyway.
And it is going to cost you as well....... plating isnt cheap (not in the UK anyway)