Hmm well I did an underseal project on another Fiat last winter.
I invested in a product called Eastwoods Advanced Chassis Black, it is more of a rubberised type coating like the 3M product.
Applied easily and so far looks as good as the day I applied it. Only downside is the cost,
I got mine from Frost Restoration and it was around £25 a can which to be fair only covered two chassis rails and two wheel arches. It did apply very easily compared to Waxoyl.
Obviously all areas were thoroughly jet washed, then weak areas wire brushed / sanded to bare metal. Once this stage was completed I used Hammerite anti rust primer for 2 x coats - then applied 2 x coats of the eastwood chassis black.
Really worked rather nicely so far.