Be careful what underseal you put on some of the crappy stuff will make it worse by trapping moisture. Don't just trust anyone with this as few members have found out has caused rust to come back fast. Some proper vw Audi stone chip on high impact areas along with clear wax oil in chassis rails might be a better option than the black stuff.
im also looking to get this done to mine, a few people have also told me it might be a better option to just apply waxoil periodically as a lot of the underseals actually make the rust worse! Built hamber stuff is good but expensive
Found a place just outside my work actually, it has been highly recommended by a few folk at my work that has used him for re-sprays and body work. Went up & spoke to him seems a decent guy says he would take the appropriate panels off clean the frame from any dirt remove rust etc wait for it to dry properly and apply waxoyl. Still waiting for car to come so will let you guys know then and go over his procedure.