Old engine oil as an undercoat is as you say messy but I think coupled with regular inspections is probably as good an undercoating as anything you pay money for - well all of the stuff that is sold over here in Europe anyway. The main benefit of old engine oil as I see it is that you can quite easily remove it if you need to...

Other possibilities are waxoyl and Dinitrol

Check out this recent thread:-

Reviews on this rust prevention product

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!