I have a 51 F1. it had a great paint job once upon a time. little bits of rust were starting to work through on the front of the fender. I'm in Hawaii so everything rusts here. anyway, when I started cleaning it up from the outside I realized the underside of the fender was where the rust began and went all the way through the metal. it was just paint holding it together in some spots. now big repairs

So my question is for those of you that drive your trucks a lot, what is the best way to protect the under side of the fenders?

 

Good paint after the rust has been removed , then bed liner , to protect from stone chips

 

Thank you much Dr. Smith. I will give that a try.

 

Please correct me if I am wrong but aren´t bedline a plastic??

I would suggest a bitumen based underbody coating.

Like this one for example

Bitumen Based Undercoating - MOTIP DUPLI GROUP

Or a Wax or Bitumen/wax combination

Underbody coating | Auson

 

Fiberglass fenders and running boards; works for me.

 

Works for me also. I did put three coats of bedliner on the underside for chip protection.

 

fiberglass would be great too.
Jimcar, I had a standard undercoating on it originally but the rocks and such that the tires fling out have made holes through the coating.

 

Spray on hot bedliner or Lizard skin or clear chip resistant paint topcoat like is used on lower section of some cars if underside is painted, but first remove all rusted sheet metal and inner doublers (F1 fenders are famous for rusting between outer skin and reinforcing doublers on the inside). Use weld thru primer on all inside and contact surfaces when patching. Finally after patching I would wipe seam sealer around the edges of all doublers. See my F1 fender rust repair topic to see how we repaired the common rust areas on a front fender: https://www.ford-trucks.com/forums/1...g-try-2-a.html

 

That's the ticket. I define "good paint" as KBS Rust Seal (or equivalent). Scuff the Rust Seal prior to the application of the bed liner. I use 3M 7277 or Upol. Both can be painted the color of the truck with proper prep...scuff and an adhesion promoter. Most people don't even notice when the underside of fenders are black and not the color of the truck.