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Ask Dave: What sealant should I use between coach and chassis?

Dear Dave,
The caulking is starting to yellow between the coach and the Chevy chassis on our Class C motorhome. I want to recaulk this with something that doesn’t yellow but stays white. What sealant should I use? Silicone or Dicor or…? Thanks, Dave. —Jerry, 2021 Thor Chateau 22b

Dear Jerry,
The transition panel between the coach and chassis is commonly call a “wing wall” in the RV industry. It is typically a piece of plywood sandwiched with interior paneling and exterior fiberglass. It is typically fastened to the chassis with L-shaped brackets. Some manufacturers use butyl tape, which is a putty-type sealant at the joint with an exterior silicone or other sealant.

Since your unit is a 2021, I would suspect the sealant you are referring to is the original and the yellowing is due to UV degradation. This is common with the average sealant used in Class C units.

Try cleaning it first

If the sealant is still in good shape other than the discoloration, try cleaning and treating it with a bleach solution. Put a little Dawn dish soap (blue) into a pail of warm water and use a small brush to clean the sealant. This should get most of the dirt and any mold/algae off the sealant. Then put ¼ cup bleach in a gallon of water and dump into a spray bottle. I have one of these with me all the time to disinfect water faucets and help sanitize the dump station and hoses. Spray the sealant thoroughly and let it sit for 15 minutes or more, then scrub with a brush and rinse. You might be surprised at how well this works!

Another option is to try 303 Protectant, which I have used over the years to bring back the clear coat shine of fiberglass and plastics. I have not used it on silicone, as mine has typically started to crack and separate well before discoloration and needed to be replaced.

Resealing with proper sealant

Since the sealant area of your wing wall runs vertical, you will need a non-leveling lap type sealant so it will not run down the transition area. It is important to use a sealant that is designed for the material you are attempting to seal. In this case, it is typically fiberglass on the wing wall and painted metal on the cab. Winnebago has great sealant guides on all their products available on the website. Here is a sample of a Class C similar to your design.

The two sealants they recommend (C or E) are a non-leveling RV silicone 311 that can be purchased at most RV dealers or parts suppliers as it is not a Winnebago proprietary brand. You can also use the Dicor product which is a non-sag cap seal part number 351CSW-1, which is the white product and is UV resistant.

If you choose to reseal, it is important to remove the old sealant first. I have always found a heat gun on the low setting and a plastic putty knife works best. It softens the sealant and the plastic does not gouge the wing wall or cab. You don’t need to remove everything—you just need a smooth surface to reseal. After you reseal it, I would even apply a UV protection such as 303 Protectant once a year.

Another option is Eternabond, which is a tape that is very popular with RV DIYers, although this does require some good prep work and patience to get it on straight and wrinkle free.

Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club.

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