I'd agree. Since I had my GTV, which was spankery new, I've changed my mind about underseal. It's good for covering up repaired sections of the floorpan, since it's thick and oily and the repair is inevitably lumpy and vulnerable to repeat attacks.... but if I had a pristine car I would just keep the outer surfaces "clean", so that any rust can be seen.
You can waxoyl the inside cavities and box sections.. (inside the rear wheel arch/three-quarter panel is the most crucial).
Just use brushed-on smooth Hammerite on the crinkly finish painted suspension parts... (leave the alloy, or paint it silver).
There's nothing to be done with the pressed metal sub-frames if you want to keep the original finish. If it was me (but I have OCD
) I'd take them off while they were still shiny and get them epoxy coated and then stick a further layer of satin finish paint on the top.. but you can probably do good just by brush-painting Hammerite around the welded joints and seams (the finish always falls off there) then aerosol-spraying over the whole lot and the flat/smooth sections... to get a smooth finish.
The "Tipo" cars' bodies were 80% galvanised/high-carbon steel... but the one bit they thought didn't need doing was the floorpan..
so you only need to worry about that.
The rubbery Shutz stuff either cracks with age/mileage or it isn't quite strong and flexible enough to stay attached over the seam between the floor and sills.
On a few cars I've had of that age, the Schutz stuff peels away from the inner sill seam... I think as a result of corrosion getting underneath.. but it could just be the Schutz curling up with age and letting the moisture in, which starts the process.
Anyway, if the seam is leaking, the Schutz will lift and then the moisture spreads along and under it.. rusting the metal as it creeps along and detaching more and more of the Schutz... etc.
On the Younger Mrs S. Cinquecento, Titch.. I experimented with how I'd keep that rubbery Shutz stuff "intact". I cleaned out (brushed with soapy water and Traffic Film Remover etc.) one of the rear wheel arches (which don't have a liner in the Cinq') ... filled in the nicks and marks (use very rubbery body-filler resin with not enough hardener in it, to make it bendy rather than hard) then painted it with a regular Hammerite primer and then paint of the same colour as the car (Titch is yellow).
That goes on well.. the pimpled finish of the Schutz masks any brush marks but the surface is not hidden behind layers of heavy coating.. so it's easy to see if the metal starts to corrode or the Schutz lift (which would have to be from behind, for some other reason).
The extra finish will protect the Schutz and of course by brush painting the surface you're also filling any nooks and crevices which the factory paint job may have missed..