I read it out of interest, as I'm in the process of trying to rust proof mine. Using Dynax S50 and extending the probe, I've managed to do the full length of the sills, with access through the seat belt fixing point hole on the sill ( can see the wax running out of sill drain holes) also have done the A posts and all the chassis sections underneath. everything else underneath has had the rust treated and repainted, then sprayed with chain lubricant, which doesn't wash off easily. When the warmer weather comes, underneath will be sprayed with black Waxoyl, as l already have some, I use a Poundland pressurised garden sprayer, heat the Waxoyl and thin with 10% white spirit, works really well and no point cleaning the sprayer afterwards as it only cost a £1 !
As my arches have not started to rust through yet, I've still got to remove the rear cards and speakers so I can treat them with Dynax, also lift all the carpet to do the floor. I've also done under all the wheel arches, including have the rusted through metal on the rears replaced by a local body shop, replaced the rusty screws and washers with stainless, so they'll be easier to remove in future. My intention is to redo it all every year, as that's what I did with my Classic Mini and in 5 years the rust didn't get any worse and it had a lot more than the Puma.
It takes a fair bit of time, but doesn't cost that much compared to having it done professionally.