I finally have one of my new(er) inner fenders cleaned and painted with 2 coats of rust reformer. As I was starting on the second one (drivers side), I started thinking about what should I coat the underneath side with. Should I use epoxy paint, spray cans of undercoating, or rubberized bed coating? What has worked good for your truck? Or what would you do different next time? This truck most likely will not see any hard off road use. In fact, only very little dirt road use. But it does rain a boat load here (26 days out of 30 last month).

 

For stuff that sees road debris exposure (wheel wells under side of the body etc) I like the asphalt based undercoating as it is easy to touch up and never really hardens so it does not chip crack or peel, that is my preference.

But here vehicles can see ambient temps from -40F and below to 90F plus on occasion and are exposed to cold temp sand blasting and exposure to calcium chloride, as tens of thousands of tons of calcium chloride laced sand is dumped on the roads here in winter.
Plus some the soils in the region are very low PH, in the 4's (Extremely acidic to very strongly acidic) and the road gavels have a fairly high salt content (of course that is the area where most of my gravel road travel occurs ), so lots of travel on dirt roads can really tear up a body and accelerate corrosion in short order.

Over the years I found that the asphalt undercoats perform the best they just don't stand up well high pressure washing but are the easiest to touch up. For body cavities auto body wax or cavity wax is the best stuff to use. it comes in different formulations from thin like water that just leaves a thin surface film, to thick like beeswax when it sets up that will tend to creep the rust inhibitors across the surface of the panel for years after install. It also repels water just like wax does. It is just not good for areas exposed to road debris or pressure washing as it will blast off.

That is what I have found works best in my area. Being in WA with lots of rain and a fairly mild climate and not seeing cold weather gravel road travel rubberized undercoating might work best as it more resistant to high pressure washing than the asphalt based stuff, Here I have found the rubberized stuff tends to crack and flake off in really cold temps when exposed to road debris.

I'm not a fan of bed coating that sees exposure to road debris impacts in cold weather as they will over time crack and allow water penetration and then corrode underneath especially on something rather thin like an inner fender. Unless you are going to put it on really thick. So the asphalt based stuff that never really sets up, I have found works best here.

 

If you can find undercoating with bee's wax it will hang on very well. Fluid film works great, you'll need to apply annually for best results.

 

I was leaning towards that type of product, but it's always good to hear what has worked for other trucks. I'll start searching for undercoating with bee wax, or go with the asphalt based stuff. Thanks guys!

 

The bees wax stuff will not hold up long term in a wheel well, it will get blasted off eventually and it does not hold up to pressure washing. It is basically the same stuff as some of the thicker body cavity wax's.

 

OK, I'll use the asphalt based stuff in the wheel wells and the wax stuff inside of the doors. Thanks again.

 

I use Fluid Film on my daily drivers. I put it on twice a year(spring and fall). Typically it gets washed away around the rear wheel frame area so that is a spot I concentrate on most. I cut it with a bit of used motor oil and use the spatter gun they offer. Make sure to park in an area that you can put up with oil stains. It will drip for a few days til it sets up good(at least if you cut it with used oil).

I pull door panels off and spray it on heavy, along with inside tailgate, front fenders, back of hood. Just be ready for a drippy mess for a while.

 

Good to know Freightrain!