Dear Dave,
I purchased my 32-foot 5th wheel RV new last year, and fixed several issues myself. I tried to get a warranty on the underneath metal plate of the RV, which was very rusty. When I asked Camping World to undercoat the chassis, they would not. Is it practical to use spray undercoat and do it myself? Do I need to sand blast it first? —Roger, 2022 Keystone Montana High Country 295RL
Dear Roger,
What you are seeing underneath is typically surface rust, as most frame manufacturers simply do a light paint and not a true powder coat or electronic dip tank coating. Even the motorhome chassis manufacturers have the same issue. I have a friend that has an Ikan Renegade that has the entire underbody completely covered in rust and looks 100 years old. Even though it is not a structural rust, it still looks horrible for the amount of money he paid for the unit. He is having the entire underbody sandblasted by a local body repair facility and repainted.
When I was working at Winnebago we did offer an undercoating option, but it was not ordered much as I assume the cost was fairly high for something you did not see or get to actually use. I also know that it created quite a mess when something had to be removed or worked on.
The issue with rust
The issue with rust is not only does it look bad, but it can also have an effect on any grounds that might be on the frame, such as the steps if they are electric, or other components.
Cleaning the rust off the metal is important before applying an undercoating or simply painting it. However, I don’t think you have to sandblast the entire thing. There are several spray on products that are good rust removers such as Fluid Film and Corrosion X.
Chemical-based rust removers work fast but can be very caustic. So be careful to spray just the metal components and not areas that have plastic and painted surfaces where you want the paint to stay. Petroleum-based removers such as WD-40 penetrate the rust and allow you to wipe it off and leave a protective film, which I believe would not be good for undercoating or repainting.
Rust Converters actually transform rust into a surface ready to paint and might be the best alternative. Corroseal is a nontoxic product that claims to remove the rust and prepare the surface for painting. I have tried both Fluid Film and Corrosion X and both need a heavy cleaning after as they leave a film similar to WD-40 but did a very nice job of removing the rust.
Let’s see what other readers have found.
You might also enjoy this from Dave
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Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”
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We bought a new 5th wheel in 2008 & 2 years later I decided to repaint the underside, since we planned to keep the 5th wheel for 15-20 years. I scraped all the rust off & applied Rust-Oleum rusty metal primer & then 2 coats of a black finish paint. It’s now 14 years old with 200K miles on it. It looks much better than it would have if I hadn’t painted it.
Most DIY and some commercial undercoatings harden after a while. Then they start to crack and break away. This allows trapped moisture to attack the metal underneath.
We have a local independent rustproofer that developed a coating that never dries and flows into gaps and crevices and has a rust inhibitor. It is a mess after application since it drips for a while. I had my 2015 Silverado done and there is no evidence of rust anywhere. I live in Western PA where road salt is used during the winter.
No matter what route you go with, you will have to remove as much loose rust as possible first.
I bought our 2023 Forest River Cherokee Wolf Pup in June of this year. It has a May 09, 2023 manufacturing date, so it has not been sitting on some lot for months. I noticed rust on the frame in several locations. I used a harsh chemical rust converter, let it sit overnight, then rinsed. After another day I used a Rustoleum Rust Reformer spray paint in the automotive department of Wal-Mart along with standard Flat Black spray enamel. It is looking like new again. for once I caught it before it got really bad.
I had some surface rust on my 5th wheel on the long structural I-beams that run down the sides. I have had good results with Rustoleum Rust Reformer and similar products. I wire brushed the rust to get off the ‘blisters’ and then sprayed the rusty spots. It forms a barrier to future rust and since it is black, I did not have to paint over it. I also used it on a car fender, which required much more careful prep, and after finish painting and clear coating got good, lasting results.
The classic car community uses a product called POR-15. It is a brush on coating that holds up well. Prep involves wirebrushing off the heavy stuff and applying their prep solution. Not a difficult job but it can be messy when working overhead as you would be doing an RV frame.
Much ado about nothing, as the saying goes. Look at the underside of your tow vehicle .. same situation as the trailer.
I have seen frames rust through but only in locations where road salt is heavily used or seaside locations.
When I lived in Michigan a person could have a new or used vehicle treated with Ziebart but it was only warrantied on the new vehicles. It was (is?) an expensive procedure and required lots of strategic hole drilling as well as some disassembly and reassembly to get the goo into all the nooks and crannies. Unless something like this is done you are putting lipstick on a pig, as they say.
Personally I would want both some distance and a respirator rated for the chemical vapors involved in undercoating. As I have neither a pit nor lift for the trailers, if I thought this important, it would not be a DIY project.
I wire brushed the heavy rust areas and blisters where I could get to them and lightly sanded some of the frame areas. Then applied RustOleum Rust Reformer then painted it with RustOleum Bright Galvanizing Compound. (It is a “zinc-rich coating”), (#20066 75848) Looks great after several years and might even reflect some heat from hot pavement and exhaust.
Another excellent product from RustOleum is their “Stainless Steel” finish which I have used on yard equipment.
Since you will mostly likely be upside down on your back- be sure to use personal protection, i.e., face mask and eye protection. A hat would be good as well as hand protection. Easy clean-up with mineral spirits, soap and water.
Thank you, Dave! This is just what I need to know right now. I’ve been detailing our MH this week and found a few bits of rust on the attachments of the struts to the storage bay doors. I placed an order through Amazon. Thank you!
I have a garden shovel that is 40+yrs old and is still strong and has been rusting for decades. Surface rust is not a big deal other than looking bad. It does kinda hurt after we paid so much for the rig but RVing is an expensive hobby.