Monday, December 4, 2023


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.


Chuck Woodbury
Chuck Woodbury
I'm the founder and publisher of I've been a writer and publisher for most of my adult life, and spent a total of at least a half-dozen years of that time traveling the USA and Canada in a motorhome.



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Neal Davis (@guest_113458)
2 years ago

Great information; thank you!

Grant Graves (@guest_113417)
2 years ago

Thanks for the very informative article. The comments are enlightening as well.

cee (@guest_113395)
2 years ago

I have always opted for lighter colors for anything that has to endure the sun due to oxidation and interior heating. Those mostly black MH’s may look elegant and sexy when new, but everything ages; some more than others.

Brenda Childers (@guest_113394)
2 years ago

Good article and very timely. My husband and I own a 2004 Tiffin Phaeton that we love and like you we are unhappy with the appearance of the striping and clearcoat. We too are unwilling, at this time, to go into debt for a newer model with full body paint. So, we’ve got an appointment for June 1 to have Double-B in Red Bay, AL, remove the old decals and paint new stripes. They quoted us a ballpark figure of about $10k which is much less than what we’ve been quoted to get the same thing done here in Washington state. We had already decided to replace the darker stripes with a lighter color and reading your article confirms we’re on the right track. We’re excited to have an adventure…Red Bay here we come!

Thom (@guest_113369)
2 years ago

New coach has quite a bit of black. It’s the one we wanted. I would not have passed on it just because of the black. Luckily, covered RV port at home and minimal south exposure.

Bob P (@guest_113358)
2 years ago

I had two black cars when I was much younger and admit it is beautiful for 1 1/2 days after it’s washed, then it needs washed again. RVs use to be painted white or off shades of white which made a lot of sense since the lighter colors reflect the suns heat better. Then some modern day executives got a race horse deal on black or almost black paint and suddenly the market is flooded with these beautiful but maintenance demanding RVs. Personally I wouldn’t consider one because of the heat absorption, plus I’m to old to wash my RV and it costs too much for someone else to do it. But they are beautiful for a day and a half before the dust shows on it.

Tommy Molnar (@guest_113459)
2 years ago
Reply to  Bob P

You are SO spot on, Bob!

RV Staff
2 years ago
Reply to  Tommy Molnar

“SO spot on…” Tommy is being punny today. 😆 —Diane at

Engineer (@guest_113355)
2 years ago

It’s amazing how many RV issues mirror those of boats…crazing, checking of dark fiberglass has been an issue for countless years. In some cases it because of defective manufacturing but most are caused by lack of owner preventative maintenance. And today very few owners actually wax their fiberglass coaches but rather turn to the magic spray and other mumbo jumbo stuff being hawked on line. This approach is a big mistake if you plan to keep it. Fleetwax paste wax is the real solution IMO.

mike henrich (@guest_81845)
3 years ago

I just completed restoring the gelcoat and painting our 2005 RV. The left side had thermal checking. I sanded the checking and applied Bondo glazing to smooth out the repairs, then repainted. The checking was only under the previous decals so that made my job easier, since I painted the same design back on.

Tom Smithbrother (@guest_81817)
3 years ago

I kept expecting to see what coating /waxes would minimize or stop the damage. But the article was about how to deal with it after it is already been damaged. I wish the the article would have been about how to avoid/minimize it in the first place.

David Telenko (@guest_113365)
2 years ago

Tom well he did, “The only effective way to prevent thermal checking is to keep your RV completely shaded from direct sunlight when it is not in use.” Floyd also said ” 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.”
hope this helps

Don Baker (@guest_71389)
3 years ago

I also have thermal checking on my 2005 coach. You did not mention the coach name but mine is a Country Coach. The problem stems from a supply of fiberglass panels from Dow Corning that had the wrong catalyst used in manufacture. CC successfully sued Dow and won a settlement of about $850,000. Unfortunately the CC owners had sold the business and the new owners spent all of the settlement on office upgrades and who knows what prior to filing for bankruptcy, leaving the coach owners out in the cold.
I have been told that once the fiberglass has recatalyzed with the heat it will not do so again. I have owned my coach for about six years living most of the time in Southern CA and have not seen any increase in the checking.

Donald N Wright (@guest_71373)
3 years ago

This is interesting, and possibly overlooked by owners and manufacturers.

David Norton (@guest_47295)
4 years ago

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 (@guest_47218)
4 years ago

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 (@guest_47209)
4 years ago

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 (@guest_47207)
4 years ago

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

Tom (@guest_47165)
4 years ago

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 (@guest_47127)
4 years ago

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 (@guest_47085)
4 years ago

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.

Don (@guest_47574)
4 years ago
Reply to  Tom Moeller

There are several different types of Evercoat. Which would you say is best for this application?

There are a huge number of very expensive RV just a few years old. I see a great opportunity for someone here.

The research I did on this subject said it started in the early 2000s and wasn’t corrected until 2008 when a change was made to the backer for the gel-coat. Unfortunately my 2004 Safari Trek is a victim.

Carl Jones (@guest_47073)
4 years ago

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

marty chambers (@guest_47069)
4 years ago

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.

Tommy Molnar (@guest_71399)
3 years ago
Reply to  marty chambers

You only have to own ONE black “anything” in your lifetime to learn this lesson.

H Goff (@guest_71450)
3 years ago
Reply to  Tommy Molnar

agree – been there, did that, no more black…

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