The Stovebolt Site Forums The Shop Area Paint and Body Shop What's that, under the Fenders??

Woke up this morning and was thinking about how I will treat the underside of the fenders. If I was doing a show truck I suppose I would paint it with as much care as the outside. Since I’m building a driver I will now doubt use a different approach. Several choices come to mind, so I would like to get some other opinions. I haven’t been to a car show in 45 years so I don’t know what all of the options are.
One; leave it bare, after all it probably came from the factory that way. But since they will have been stripped by some means, ie., chemical, media blasting, scrapping the surface will be much more vulnerable to rusting.
Two; Paint with an e-coat, or prime and paint with color of choice.
Three; undercoat! I see a lot of areas where this truck was undercoated at one time. This looks like it protected it very well. Then again I have heard stories of water getting under and lifting the undercoating and forming a pocket that holds the moisture.
What did some O’ you guys do with the under side of your fenders???
Denny Graham
Sandwich, IL

DG, I'm considering using a bed liner material when I get to that point.

Dan

Mine got a good primer coating, then spray can undercoating. It's held up pretty well, especially since the fenders haven't been put on the truck yet....

The factory assembly manual does call for undercoating under the fenders and cab of these trucks. I was suprised to see black paint in pretty good condition in the few spots where it remained under my front fenders. I know a lot of folks these days use bed liner like undercoating. I believe the theory is that the rubberized bed liner will not seperate like old school undercoating and also acts as a sound deadener. I too have heard that undercoating can trap water, but this can only happen if the undercoating is not properly and fully bonded to the surface. Some of the rod guys glue dense foam pads under the fenders, but this is more to stop paint damage on the exterior from rock damage than to prevent rust. Just a good coat of expoxy primer and paint will do fine as long as dirt and trapped water are not allowed to build up under fenders.

Denny, I'm in the same situation as you.. I would sandblast it all, spray it with epoxy primer and then spray a black base coat over it. Then you can get this spray stuff from the paint store that's a rubberized material to spray under there.. that way, when your tires throw rocks up to the fender, it won't ding it..

Just my 2 cents....

I would use the 3m rattle can fender undercoating. I used it on my Jeep and it held up great. It is a bit more expensive than truck bed liner material but it bonds to paint a lot better and looks better.

Interesting Dan, nothing would stick to it then, as it seems that everything slides all over in the beds that are treated with it. Are the home products, is. Roll-your-own as good as the commercial applications?

Well, that'll do it Tom, don't put the fenders on and they can't get dirty/rusty.

Nifty, yah, I was looking thru the Assembly Manual and noticed that they called out undercoating for the cabs and back of the fenders from the factory. Since I’m shooting for a restoration I will probably go with that, however I may apply it to a little more than they call for in the manual.

Josiah, is that the stuff that stays kind of soft, kind of waxy?

Denny Graham
Sandwich, IL

por-15?

This is the stuff I've used in the past. It dries solid, is kind of a rubber with a matte surface. It has a fairly smooth surface so nothing sticks to it. Bedliner tends to hold dirt.
It is also paintable within a window of time.
3m undercoating

Tks, looking it up as we speak!
DG

I also used black por-15 under the fenders

I also used Black Por-15. Most of the underside of my '49 is covered with this stuff.

POR-15 is fine but it does chip from rock impact. It takes a flaming meteor to chip off the 3m stuff.

without a doubt, use some body coating, mine has it all over underside from new, as an option, and the truck still remains as new, in all areas --not pretty, but saves you restoring again, and again, unless you like restoring that is.helps with the sound deadening aswell.

I'll have to take your word for it Stevie-t, understand it rains 363 days out of the year over there, ha!
I believe that's the conclusion that I have come up with also. My '50, 3604 has factory undercoating, as per the Assembly Manual, in the designated places, and it's still soft and waxy in spots, (stiff, but you can scrape it with a putty knife). Where the frame and body was coated it looks like new metal underneath, absolutely clean not a trace of rust, else where I have mild surface rust.
A couple of body men that I know, that build street rods, highly indorse Mar-Hyde Rubberized Undercoating (Bondo Co.), they claim it’s the only stuff to use. Don’t see why 3m Rubberized Undercoat wouldn’t be just as good, but they’re the experts with the experience.
So, I think what I will do is the rough (i.e., just short of leveling) body work, send to Redi-Strip, epoxy prime as soon as I get them back, do my finish body work and paint build. Lastly, use one of the rubberized undercoats on all the underside tin.
This is not gonna be a daily driver, and I’m putin’ her up on blocks when the bad weather hits. I might not even drive her in the rain, unless I get caught by a sudden storm, I figure she’s seen enough bad weather in her 56 years.
Thanks for all the opinions guys.
Denny Graham
Sandwich, IL

Hey Denny, you have a very good plan there but I would do the underside before the final paint on the truck (speaking from experience here) because of the mess that 3-m undercoat or Mar-Hyde makes during application.

As careful as you can be won't be careful enough and no use to risk a high dollar paint job due to sequence of procedure.

The epoxy primer will for sure keep any rust from forming and will bond well to any of the rubberized undercoatings. Best of luck.Hobert

I'm fixing up a 46 1/2 ton truck. Much of it was painted by the privious owner with John Deer under carrage paint. Semi flat, very tough and the guy told me it was easy to use. I have been banging and scraping on it and its good thick paint.

Jim