Waxoyl is not a rust converter, just extremely good at slowing future damage by acting as a waterproof barrier, so ideally it should be a final stage after rust removal and conversion. If you get it done professionally, the 3 stages will cost around 1200 ( that's what I was quoted on my Jag anyway). Its a messy job to do yourself and difficult without a lift.
It's better in sealed sections like sills and doors, than on open areas, as it wears off easily, so underbody needs coating every few years to maintain cover.
Many people prefer Dinitrol to Waxoyl but I don't know the differences.