Gelcoat Cavities and Cracks - Fiberglass RV


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Old 06-08-2010, 11:42 PM   #1
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Just when i think i have a handle on the projects my Burro has thrown me, I discover a whole new set of issues that lead me back into the weeds. I was working on the top vent replacement today when I noticed that the darker colored fiberglass on the center seam corresponded with the chips on the roof gelcoat. Previously i assumed that the darkness was from a leak that traveled from the vent through the rat fur to a resting spot and molded. Now I believe every chip on the roof has let water in to damage the glass and turn it grey. When you poke a crack from above it gives just like a cavity until you find sound material surrounding it.

I literally have hundreds of these cavities on the roof and have multiple spider cracks on various surfaces. I tried to patch a few of them with gelcoat but have little patience to repair the whole exterior AND interior with this time consuming method. I'm considering a light bondo over the cavities and a sand / paint. Which do you think will be more work? I hate half way painting things so would want to take all the windows out and remove all the hardware. Gelcoat seems to take way longer to sand but is a very nice finish in the end. Is there an easy way to fill all these holes? What do I need to do to repair the water damaged fiberglass? Do I need to lay patches of fiberglass on the interior to reinforce the rotten areas? Should I be concerned about the spider cracks as much as the cavities? As usual, I'm feeling a little bit overwhelmed and could use a little advice. thanks!
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Old 06-09-2010, 01:05 AM   #2
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I'll be curious to follow this as I have some older gel cote repairs to address. It looks like the cause of your cavities and spider cracks are different. I wonder if it might represent bubbles in the structure that weren't properly pressed out, and as the gel coat thinned and oxidized, water began to get in and develop a dark fungus? Does full strength bleach (use gloves) clear out the dark patches?

I think the spider webbing is structural. The glass can flex, but the thin gel coat is harder, and it can't flex as much, leading to these patterned cracks. Unless it's impact damage, it was probably caused by vibration or flexing of unsupported areas or stress points, and it would eventually return if it isn't corrected somehow, even if you patch it up.

I think paint is a lot more flexible than the gel coat is. I'm definitely no expert on this stuff, so I will shut up now, and listen!

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Old 06-09-2010, 06:10 AM   #4
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An excellent product for narrow cracks is:

Captain Tolley's
Creeping crack cure.

sold at most marine supply stores ( try West MArine ) a 2 ounce bottle will set you back over $20, but a little goes a long way.
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Old 06-10-2010, 06:54 PM   #5
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I'm curious as to how you'll fix those chips. I have a few random chips in my gel coat but no where near as many as you. I've been picking at the chips to remove the loose gel coat, and quick fixing it with 2 part marine epoxy.
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Old 06-10-2010, 07:38 PM   #6
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I had a sailboat with 'bouy rash' down one side from an unmarked bouy near the North tower of the Golden Gate Bridge.

I used a two part gel coat repair kit from West Marine.

I mixed color with the base part and put dots on the gel coat until the colors matched. Then, I added the hardener.

Fill the pits and put a piece of mylar tightly over the repair. The mylar will not stick to the gel coat and will make a very smooth surface. Remove the mylar after the gel coat sets.

Let it set for a couple days.

Sand with 300 then 600 rit. Buff.

I don't remember the time, but, I let it sit for a week or more before I waxed it.

When finished, it was hard to tell where the repair was.
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Old 06-10-2010, 09:31 PM   #7
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I'm a bit wary about responding to this query but will give it a shot. First I used to make my living repairing fiberglass boats in California Hawaii and in NZ. I'm retired and still own the same sailboat which I cruised in. So... I'm going to make some less than complimentary comments about our little eggs. So here are my thoughts about our little eggs....

Our little eggs are built the same way as fiberglass shower stalls were built of that same era. They were quickly laid up using a chopper gun with small areas of mat applied where extra strength was deemed necessary. Commonly when a fiberglass molding was made the gelcoat was blown in and followed by a second darker colored gelcoat layer. This second layer is often called a "veil coat" This second layer did several things: first and most important, it made it easy for the trapped air on subsequent layers to be seen and rolled out as voids appear as white. Two: it provided opacity which provided some privacy to the owners of the otherwise translucent fiberglass. And three it helped modify the color of the first layer of gelcoat (a blue layer behind a white made it brighter and less prone to yellowing...just like a drop of blue paint in a gallon of white makes it brighter). Ok, so the builders of our eggs saved some money and left out the veil coat. And as a result the thin layer of blown in chopper gun fiberglass was often not fully rolled out because the person doing the rolling couldn't see the bubbles and uncompressed areas. Rolling out is where steel rollers compress the fibers and make the otherwise porous strands a cohesive whole.

Improperly rollered in fiberglass makes for unsupported corners and details. We have probably all seen them, the gelcoat breaks and there is a void before one encounters fiberglass. They are common on corners and edges. And when the fiberglass strands are not properly rolled in one gets the spongy areas with little strength. That I suspect is the cause of both of your problems: the pocks and the softness/sponginess.

Repairs are going to be making the best of an unfortunate situation. Depending upon how loose the strands are will determine whether you will be able to press/force/squeegee in a polyester resin filled with something like micro balloons to the consistancy of light cream. This would be followed by a layer of 3/4 once mat wetted and rolled out on a sheet of cardboard before placing overhead. You would be working against gravity and intimate contact between the old and new layers will be problematic. Vacuum bagging would perhaps help but you would have to seal the exterior otherwise one couldn't draw the vaccum. Otherwise simply staying put and rolling back the new mat as it wants to droop would be your best solution.

Once it has gone off one can repair the gelcoat voids from the outside. If you drag your fingernails over fiberglass one can detect voids by the distinctive sound change when the nails go over a void. If you ever see someone scratching the decks or side of a fiberglass it is not because the boat has an itch they are probably surveying the boat. I would suggest breaking out the discovered voids and filling them at the same time. Unfortunately this is labor intensive work. Another way to locate voids is to place a strong light inside the trailer on a dark night the voids bubbles etc appear as dark areas. Same for bruises and bumps or places where water has entered the fiberglass.

Something to keep in mind: if you are wanting to beef up an area. These shells are thin and if you apply a heavy layer of fiberglass (say to make a internal stiffener) be aware that often the stresses of the new fiberglass as it cures will cause distortion of the shell. Walk down a dock and often one can see where the internal bulkheads on a fg boat are by this distortion and those hulls are many times thicker than our eggs.

Sorry to run on so but this is not an afternoon project if one wants to get it done as best as possible. On the bright side these lightweight shells are not supposed to cross an ocean. My U-Haul is cute but honestly for not much more additional weight they could have built a much stronger shell.
My bet is they didn't hire boat builders as fiberglass workers, perhaps not even builders of FG showerstalls.

Hope this helps,
David A. L.
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Old 06-10-2010, 11:11 PM   #8
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Quote:
I'm a bit wary about responding to this query but will give it a shot. First I used to make my living repairing fiberglass boats in California Hawaii and in NZ. I'm retired and still own the same sailboat which I cruised in. So...
David, I think [b]you are just the guy I am looking for. Welcome to the Forum!!

I am in the middle of a big rebuild of my '86 Scamp and could use your best advice on the roof I got.
Trouble summary: Tree fell on it, big cracks, plus lots of spidery stress cracks. I am sealing up many unwanted screw and rivet holes, and I built up the rim around the exhaust fan, plus there is the usual oxidation dullness of fiberglass. These pictures show what I mean.


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The finished job should look superb (naturally). I do admit to rationalizing for low standards if necessary, particularily when I am doing the work, and a desire to get fantastic results without spending the big bucks. Therefore, the job will be done in my backyard, under my tent. At first I was going to paint it. Now I want to repair the gelcoat instead of painting the whole thing. I worry about the gel covering (hiding) the blotchy fiberglass patching I have done.

Your kind and generous advice on materials and sources, as well as pitfalls, would be wonderful.
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Old 06-10-2010, 11:30 PM   #9
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Excellent post David!
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Old 06-11-2010, 12:46 AM   #10
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Roy thanks for the kind words, however, I do not wish to present myself as "Mr. Fiberglass" because I'm not (nor do I wish to be!). I have a certain level of skill with the material but there are many techniques that have been developed since my last major fiberglass project. That being said, old tech still works and where I can be of help I will endeavour to assist.

Myron, I have personal prejudice against bondo. Great stuff for bonding to metal and I have used it as a transition layer bonding rusted out sheet metal to fiberglass. I would suggest you look into either a product called Q Cell which is white/cream colored or micro ballons which can either be white if made of glass or red brown if made of phenolic resin. Both are mixed with polyester resin and will make for easier sanding and feather edge nicely. As for use of epoxy a good rule of thumb: epoxy sticks and bonds to fiberglass, fiberglass doesn't to epoxy. Yes, there will be some that claim they pulled it off, but like polyester on wood over the long term it doesn't work well. Repairing with such a product as Marine Tex basically closes the door to recoating with a new layer of gelcoat. No knocking Marine Tex it is great in it's place, however, I prefer something called Splash Zone which is a two part epoxy. It's very easily worked/shaped and must be mixed underwater and worked with wet hands. If I was looking at painting (as opposed to gelcoat) it is what I would use for repairs such as rivet holes. I keep a two quart kit on my boat for emergencies and have what left of another in my shop for odd jobs/repairs.

Gelcoat can be mixed with a material so that it can be sprayed successfully. When I re-gelcoated my 33 ft boat I used a product called "Ramenel" or something close on that. A search on Google doesn't bring up anything. Which isn't too surprising as that was back in the early 80's. It was made by Ram Products and if memory serves, was mixed with gelcoat almost 50 /50 and one needed a large tipped spray gun with a gravity feed cup and the capacity to push alot of air (I use a Binks-Bullows mdl 630 which has a nylon cup and quill but that doesn't put it in the in-expensive category). Gelcoat can be applied so that it has a fairly light orange peel which if one wanted can be hand sanded to make like something out of a mold. Improperly applied it can look like someone got carried away with drywall texture. When I did my boat I was fortunate to be friends with the man who created the FG animals that hang overhead at the Monteray Bay Aquarium. His advice and assistance helped alot.

FG materials have gotten frightfully expensive since I was young and deep into it. Most of the jobs/ stuff I worked on was for strength and shape. Weight was a secondary consideration save for racing boats. These thin skinned unsupported shells are new to me. Used to be that one could get rid of spider cracks by simply sanding them out and replacing the gelcoat, with these thin skins a trifle to much and you are thru the skin. The photo you have of all the holes and the can of bondo: Where is that on your trailer? It sort of looks like the top, I have the vent off the top of my U Haul and it is nowhere's near that thickness. The roof on my trailer looks like ounce and a half mat (equivalent) at best...a real balloon.

It's late and time for me to be in bed.
Hope this helped,

David A.L.

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Old 06-11-2010, 02:22 AM   #11
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To follow up on my original post, I have decided to gelcoat all the cavities and have completed about 1/3 of them. I have gone over the entire exterior with a small rosebud carbide on a dremel to excavate every small crack to get to solid glass. Then I followed the same steps as in the second video i posted. I'm satisfied with the results but not looking forward to do the same repairs to all the holes on the interior.

After reading David's posts, I have an understanding of how the cavities developed in the first place (manufacturing defects). I too thought to myself that the shell is rather flimsy but obviously they work. Now, my next concern that I haven't addressed is all the spider cracks. They are not nearly as weak as the cavities and don't appear to be letting in much water. I thought of sanding the surface a little and skimming gelcoat into the voids but am not sure if this will actually seal all the fine cracks. Like I said earlier, it would be great to figure out a way to reinforce the weaker expanses of fiberglass with some sort of backer. No one responded to the idea of pouring polyurethane foam in to fill the space between the walls. Does anyone have any thoughts, experience or alternative solutions.

Thanks for all the great info.
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Old 06-11-2010, 09:03 AM   #12
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David - taken this morning, these pictures show front and back views of the roof and current state of things. Long spider stress cracks in gelcoat hardly visible but they exist, big time, lengthwise at the molded curves of the roof. Everything else is either a filled rivet hole or a repaired crack. I used Bondo (both short strand and smooth finish versions) so that horse has already left the barn. No going back.


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Thanks for staying up late last nite.

Kyle: My stress crack concerns are exactly the same as yours. If David concurs, I will be sanding lightly and skim coating in gelcoat there. But, which brand or which consistancy gelcoat will it be? I also worry that gold colored Bondo I used as a final fill will show through the gelcoat.
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Old 06-11-2010, 12:08 PM   #13
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Myron,
The stress cracks you indicate in your photo can be the result of a couple of factors. One is actual flexing of the fiberglass. Gelcoat while more flexible than straight resin does crack when flexed and with age the plasticizers dryout or evaporate or whatever and the gelcoat becomes more prone to cracking when flexed. A second reason is shrinkage due to age of the gelcoat. This is common in corners of such features of boats such as cockpit corners. When spraying in gelcoat it is hard to keep the coating a uniform thickness. When working an open mold those features which will be inside corners on the finished product are presented as outside corners...does that make sense? one is working in the reverse. The result is excess gelcoat is often applied to those areas. Over time this excess dries and cracks form because of the thicker gelcoat. A third possibility is a combination of age and residual stresses created when the trailer was assembled. When first assembled the gelcoat was flexible enough to stretch but as it aged finally gave up and cracked.

The areas on the roof of your burro appear to be in the hollows of the structural shape which was designed to give stiffness to the structure. As such if you cannot cause visible flexure by pushing or pulling the structure (sometimes referred to as "oil canning") they could be cause by either too much gelcoat (less likely in this case as the curve isn't that great) or some static stresses created when the halves of the shell were joined and the gelcoat losing it's elasticity with age (which is my guess although if this were the case I would also expect cracks at the bottom of the flange where the actual join between the two halves was made) or perhaps for some other induced stress (snow load etc.).

Age is not our friend and gelcoat was not expected to last forever. If you wish to repair these areas I would suggest you hand sand the cracked areas until the gelcoat is so thin the fiberglass is beginning to show thru before attempting to skim coat. Otherwise I would expect the cracks to reform in the same locations. I would suggest you keep in mind that the gelcoat is probably the only thing cracking here (unless you can flex the areas easily, the oil canning mentioned) the fiberglass beneath is not compromised as regarding strength. What with the age of the trailer any load on the fiberglass due to stresses built in should have resolved itself in permanent deformation of the underlying glass work by now. So IMHO it's primarily a cosmetic issue, keeping the finished trailer waxed so that microscopic water doesn't eventually cause problems (freezing and widening the cracks etc.) would be sufficient for most. Therein rests the "quality" issue: how beautiful do you want your trailer? So pristine that anyone even getting near it causes you anxiety? How much fun is that? Sure one can take pride in having it perfect but where does one stop? Certainly it's an individual decision and one only you can make.

As for seeing the underlying glasswork thru gelcoat: most gelcoat is high enough in solids (colorant) to make any reasonable coating mostly opaque. Like I offered in an earlier post the veil coat can "push" a color, however, in this case since there is no such coating just getting a match will be hard enough.

And speaking of matching colors, colored gelcoat fades and matching colors often becomes a matter of matching the color when the old gelcoat is wet or when it is dry. And matching white can be a real headache.

Hope this helps,
David A. L.

Quote:
David - taken this morning, these pictures show front and back views of the roof and current state of things. Long spider stress cracks in gelcoat hardly visible but they exist, big time, lengthwise at the molded curves of the roof. Everything else is either a filled rivet hole or a repaired crack. I used Bondo (both short strand and smooth finish versions) so that horse has already left the barn. No going back.



Kyle: My stress crack concerns are exactly the same as yours. If David concurs, I will be sanding lightly and skim coating in gelcoat there. But, [b]which brand or which consistancy gelcoat will it be? I also worry that gold colored Bondo I used as a final fill will show through the gelcoat.
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Old 06-11-2010, 01:04 PM   #14
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addendum:

"... or perhaps for some other induced stress (snow load etc.)."

And after some further consideration it dawned on me, perhaps someone at sometime thought that the roof could support them and climbed up on top. :-(

Bests,
David A. L.
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