How to minimize “thermal checking” damage to your RV’s fiberglass


By Floyd Mayberry
I have seen posts on various RV forums inquiring about RV fiberglass damage known as “thermal checking.” I had never heard of this issue before experiencing it and have discovered the same to be true for most RV owners. I recently went through a massive and expensive repair to correct this type of damage, so I thought I’d pass along my experience. I am by no means an expert, just someone who now has a better insight into what causes thermal checking and how to address it.

Thermal checking in fiberglass occurs when the heat of the sun breaks down the surface of the fiberglass, creating fine cracks that first appear as white flicks in the paint (see photo). Thermal checking is more prevalent in darker colors since they tend to absorb more heat.

Sun-damaged clear coat only perpetuates this issue. In my RV’s cause, I had thermal checking and clear coat damage in several places along the left side of my 2005 motorhome, where it faces south while stored at home in the hot California sun. The damage is initially only considered to be cosmetic. If, however, it is left untreated, over time it can lead to major surface cracking and eventual wall delamination as the fiberglass begins to absorb moisture. This can take several years, depending on where you live. I personally could not ignore the way my RV looked any longer and decided to do something about it.

I spent a year researching the issue while gathering quotes from various RV body shops in my area. I discovered that the process was extremely expensive since it also involved a re-paint of the affected area. My wife and I talked it over and we decided to just re-paint the entire rig with a fresh new look. This, of course, significantly increased the cost of the repair.

Repairing and repainting the entire left side of our motorhome was going to run us $20,000. Painting the entire rig brought that total up to $33,000. One might ask why we would invest so much money in an older motorhome. This was a personal decision and there were many factors involved. We love our motorhome – it has been dependable and has treated us very well. Yes, a new motorhome would be nice, but we would need far more than $33k to purchase one. My wife and I are both retired, so it didn’t make sense to go back into debt for a new rig.

SOME BODY SHOPS repair thermal checking by sanding off the damaged layers of the fiberglass, then reglazing new fiberglass over the old. Some shops remove the old fiberglass panels entirely, installing new fiberglass in their place. The shop we chose did a full fiberglass replacement. All new fiberglass sounded better to us at the time, but it ultimately was the wrong decision, at least in our case.

Styrofoam is used to insulate most RV walls: It is glued in place between the exterior and interior walls. Because of this, when a shop removes a fiberglass wall panel, they end up inadvertently destroying the styrofoam at the same time, so it must be replaced. During this process, our shop somehow damaged our interior walls, creating small tears and cracks in many places. Our interior wallboard is no longer manufactured, so there was no easy way to repair this.

Since all of the damage ended up on the lower half of the walls, I suggested the shop install a wood veneer halfway up the walls, using a wood that matched our cabinetry. I instructed them to do the same to the opposing walls even though they weren’t damaged, so that the interior wouldn’t look awkward. This all turned out OK, and was done at the shop’s expense. If we had chosen a shop that merely sanded and reglazed the existing fiberglass, this would not have been an issue.

When my exterior fiberglass panels were originally installed by the RV manufacturer, they used a process called Vacu-Bond, which bonds the fiberglass panels to the wall structure with limited ripples in the surface of the fiberglass. Repair shops cannot employ this same method since the interior walls are already in place, so they use other methods such as clamping. I was assured that my exterior panels would be straight. Unfortunately, they have far more ripples than I had hoped for.

In hindsight, had I known what I now know, I would never have had my fiberglass panels replaced; I would have had them reglazed instead. There would have been no interior wall damage and fewer ripples on the outside.

So the big question is, how can you as an RV owner prevent thermal checking from developing in the first place? The only effective way to prevent thermal checking is to keep your RV completely shaded from direct sunlight when it is not is use. This is not practical for many people, as it involves an RV garage or expensive indoor RV storage rental and it obviously can’t be accomplished by full-time RVers. Since thermal checking is heat damage from the sun, it is more prevalent in dryer climes with heavy sunshine.

You may never experience thermal checking if you live in a wet climate with mild summers. I religiously kept my RV waxed and covered it with a polypropylene RV cover while stored at home. Sadly, all of my hard work did very little to prevent thermal checking.

With all of this in mind, an informed RVer should consider paint schemes when purchasing their next RV. The darker the colors, the more susceptible to thermal checking that RV will be. When planning out our replacement paint scheme, we purposely chose lighter colors in hopes of preventing this from happening again.

Lastly, if you’ve been to an RV show or dealer recently, you’ll notice that a vast number of today’s RV paint schemes involve a great deal of black paint – the perfect target for thermal checking. I agree the black looks appealing, but I personally would not consider a black-painted RV for this very reason. Then again, I know folks that buy a new RV so frequently that their RV will develop thermal checking long after it’s been traded in on a new one! Keep this in mind if you are considering the purchase of a used RV. You might want to think twice if you come across one with visible thermal checking.


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David Norton

I also read and talked to RV Industry personnel, that this cracking is/was a production problem by the fiberglass vendor. I applaud your decision to replace the panel. I agree with your issue with clamping vs vacusealing and ripples. However, this is the only way to eliminate the cracking from coming back. Refibergass/reglazing works well with spot repairs; holes, etc, but no way you could look down a 45’ wall and not see waves…

Don Baker

We have a 2005 Country Coach Inspire that has the thermal checking over most of the coach. I have been told that there was a lawsuit with Dow Corning by Country Coach over the problem. Country Coach won the lawsuit ($850,000) but the new owners of the company promptly spent all of the money without passing it down to any of the owners of the damaged vehicles.
So far I have not had any additional problems. I try to keep coach waxed.

Diane Mc

We also live in CA & park our 2002 at home w/driver side facing sun. This issue didn’t start until about 6 or so yrs ago. Husband sanded, repainted & clear coated. However, it started to come back, on the black! Fortunately for us, sort of, we subsequently had damage to that area, self inflicted. In getting quotes from the factory to repair damage, we were told our MH was eligible for a warranty fix due to bad panels. For $15K on our part, they would replace all the panels & repaint the entire coach w/mask. W/upgrades (& new engine we did previously, old one 200K miles) we had a “new” MH for $70k. Lucky!

Jim Langley

There are RVs made mostly of aluminum because of the issues with fiberglass. One company is Lazy Daze.


Very good article, thank your for sharing your experiences. Living in the west I would not want a dark RV for the additional heat of the interior.

John Battistoni

Thank you for sharing your experience.

Sorry to hear about the various issues that occurred, but very grateful for the time that you took to share the information with others.

Once we purchased our RV, I thought about the impact of the sun on our unit. (Never had a thought that it would affect the fiberglass!) So we popped for a storage unit so, when not in use it would be protected. (My original reason for the storage facility was for concerns about the tires!).

Tom Moeller

As a retired auto body technician with 45 years experience think of thermo checking on a older corvette.A deep sanding of the affected area then applying a product called Evercoat would have been so much less financially.Actually talking to a shop that does boat repair and painting would be most educational.Not saying they can do your repair but they know products and procedures to help you with knowledge.

Carl Jones

Thank you for writing this article! Very informative and something I’ve never read about heretofore.

marty chambers

I would never buy anything painted black, car, truck, boat, or RV. Having lived in Florida all my life I have seen what happens with black and dark color items. It never made sense to me that people relished dark colors. Now I have another reason to maintain my dislike of dark colors.