Has anyone heard of coating undercarriage?

Used crankcase oil lightly sprayed underneath will do the trick.

I lived in the North salt, salt and salt was everywhere each and every winter. We don't have tens of thousands of cars on the side of the road broken down where the chassis's fell off because of rust! :B:W:B

Seriously don't sweat it. :C

There are two schools of thought and a few different coatings that may make a difference.

I have not seen it in years but there used to be a product line that was sold as "rust converters" supposed to react with rust, stop it and seal it, I can not attest to how well it did or did not work. Only aware of the existance.

The two schools of thought are as follows:
Undercoating, if done properly, when new, is great, but if you chip it and moisture gets UNDER the coating, then it is the exact opposite of great, it may make it rust faster.

That said, there used to be a company that guaranteed their work. I had 'em do a car I once owned, and yes, it rusted out,,, but well after the warranty period.

Not sure if the undercoating made a difference or what that difference was.

I come from several automotive restoration projects over the years, I'd suggest using some POR15 product - clean it, converts it and coat it. I've done this with several things over the years and it works.

www.por15.com