Must admit, i'm not a fan of underseal, water seems to get behind it and then the rust starts.
When I 1st buy an mv, I go to the local commercial steam cleaners and have the underneath steam cleaned, I then scrape off any remaining underseal by hand.
I then paint/spray the underside with standard black gloss, b&q/homebase etc etc. I find black gloss stops the mud sticking to it.
I then pressure was the underneath about 5-6 times a year to keep and mud/dirt at bay.
Of all the landy's ive owned, I have only had to weld 1, and that needed welding when I bought it.
Thats what works for me.