Why not use this on unpainted steel? Seems a great use and even more important.MPHD is the best stuff out there for under coating. I use it regularly on customer trucks in my shop. I do wrap the trucks with plastic wrap (the type on larhe rolls like you would wrap a pallet of freight with) to avoid overspray getting on the paint. I also use a product called POR15 on the spindles, front driveshaft, axles, rear differential etc due to being non painted steel. The link below is to finding it on the Amsoil website. If you make your purchase through the link provided, I become your sponsoring dealer with Amsoil and as a sponsoring vendor here on FRF, will always be here to help with anything you need.
AMSOIL Heavy-Duty Metal Protector
---------- Post added at 09:44 PM ---------- Previous post was at 09:42 PM ----------
Here is a link to the thread Rick@FreedomMotorsports referred to that shows in depth comparison testing we did
http://www.fordraptorforum.com/f36/clear-undercoat-1622/