what under coating did you use?

I did my cars underside and hard to get parts by stuff called Dinitrol. its little difirent, it will newer dry completely so it will stay soft and mess you up when you are under car ;)
tectyl is similar product.

http://www.valvolineeurope.com/english/products/tectyl
http://www.dinol.com/

BUT i like it this way if you but thick coat of hard undercoat and it traps moisture it will rust and you cant even see it.

My frend has car... Datsun 1980 and it has been driven all year around (there they put salt in rads in winter) that car is completly rust free. last time that car got undercoated was on 1990 and coating is still waxy and repels water.

first pic is frends datsuns door from inside, look how thin the paint layer in on the door. Second in my dusters underside thats after 2 years of driving