Mark , Being out where you are I wouldnt bother .. Wont protect it from chip as such, but will stop it getting surface rust, will look like crap though ..
A garden sprayer pump pack to apply sump oil thinned with a bit of diesel and spray the undersides with a good liberal coating, then go for a decent drive on a nice dusty road to seal it ..
When you get back to a more on road type area in the future, give the under sides a good steam clean and then give it a good coat of what ever you want to tidy it up with to get it ready for sale .