I'm going to try out Waxoyl done by a local company in Rochester NY. I like the one time application (with one free followup within 25,000 miles)
The Waxoyl Brand Stands for more than 55 years of experience in the fight against corrosion and is trusted by specialists and companies the world over.
www.waxoyl-usa.com
From the local company:
We use two different products. The first product is applied at 90psi to aerosolize and create a fog inside all the panels on the vehicle including the rocker panels - no drilling, we can get to all panels through existing holes. We then lift the vehicle and use that same product inside the frame and subframe. Then we switch to our second product which is a thicker black hard wax. This product will always stay somewhat tacky and is very tough.
The whole underside gets sprayed except for spinning parts like a drive shaft or your exhaust.