Repairing a tear in fiberglass - Fiberglass RV


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Old 10-06-2013, 01:05 AM   #1
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Repairing a tear in fiberglass

There have been a lot of questions about how to repair a tear in one's fiberglass shell. I thought I'd show demonstrate how I fixed repaired a tear in our wheel-well.

First, some background.

This is part of the continuing story of our doing a complete tear down and refit of our Surfside Trailer. When we bought our Surfside, the previous owner had done a really good job of camouflaging how badly rotted the floor was, going as far as to replace the entire walkable lower floor area with a carefully cut piece of plywood that lined up neatly with all the cabinet fronts, but leaving the rotted plywood to support the sides of the trailer. The rents in the wheel wells were surface-patched with a thin layer of fiberglass cloth applied over the gel coat, and thick globs of silicon caulk had been applied to the same areas inside the wheel-well in an attempt to make it watertight. This is what the damage looked like after I stripped the silicon and badly done surface repair. (I'd have shown you the "before, but I didn't realize ho big this repair would be until after I'd started cleaning the wheel-well up.)

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The damage shown wasn't caused by an accident or aging fiberglass, but by a rotten plywood floor. Even before the badly done floor repair went in the rotted plywood had stopped holding the walls of the trailer in place. The fiberglass walls sagged out and down, causing the front top edge of the doorway to crack and putting stress on the more blunt-edged contours around the trailer frame at the wheel well. Before even this floor repair was done the front top corner of the doorway cracked and sagged and the blunt-corner fiberglass contours over the trailer frame cracked and split.

Tears like these were not not the disease, but a symptom of the rotted floor. So, the first step in correcting the problem was not to fix the cracks at the door and wheel well, but to cure the disease. In this case, that meant tearing out the entire plywood floor and replacing it, which is a great job for convict labor.

Unfortunately, I didn't have any convicts handy, so I had to do it myself. Our Surfside had turned me into a condemned man, a story for a different thread.

Back to this story. Before you apply fiberglass repairs, the area to be repaired has to be stripped of dirt, oils and grease, gel coat, silicon caulk, and anything else that isn't fiberglass. Much of the work is just old-fashioned elbow grease, like pulling the globs of silicon out one little bit at a time with needle-nose pliers, but these three tools helped a lot too. They are, from top to bottom, a nimble little finishing sander (a Ryobi One cordless sander from Home Depot), an angle grinder with a 120-grit flap disk, and a random orbit sander. (From Harbor Freight. And one 120-grit flap wheel lasts a very long time; we did extensive fiberglass work on the Surfside, and used just two of them, and the second one is still going.)

Whenever you use a sander or grinder on fiberglass, you should also wear a quality dust mask. Inhaling fiberglass dust is not as dangerous as inhaling asbestos dust, but it, too, can cause respiratory damage.

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Old 10-06-2013, 01:08 AM   #2
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Choosing & cutting the right materials -- fiberglass mat or cloth -- for the repair.

Once the area to be repaired had been cleaned up and ground out, I wiped the entire area down with acetone to pick up any remaining dust and surface oil, then cut out the fiberglass cloth and mat for the repair. Because this area tore due to stress, I thought it best to build a very strong repair, 3-1/2 layers of fiberglass mat and cloth, labelled 1, 2 (the half-layer), 3 and 4 in the picture.

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The base 1-1/2 layers (numbers 1 & 2 in the picture) and the top layer (number 4) for this repair are medium-weight fiberglass mat. Fiberglass mat is a thick, felt-like material that consists of randomly laid 2-3 inch long threads of actual glass that aren't so much woven into place as they are deposited into tangled sheets. Fiberglass mat is a good base material for many repairs, and two or three layers of medium mat are all many fiberglass repairs require. Because this area failed out due to stress at the point of the tear, I felt this repair required a little more strength, and for strength in fiberglass work one turns to fiberglass cloth.

Fiberglass cloth, number 3 in the picture, is a woven, floppy and flexible fabric that's made of long strands of glass. While it is floppy, like traditional sewing fabric, the long glass fibers actually have a very high tensile strength. When woven into columns and rows of fiberglass cloth and locked into place with resin, a cloth-based repair will resist tearing and help spread the stress placed on it to the larger area around he repair. It is these properties that make fiberglass cloth a material of choice for repairs like this, where stress has caused the fiberglass to fail. You could think of fiberglass cloth as the meat sandwiched between the layers of mat that give this repair its strength.

Once fiberglass resin is mixed, it only has a limited amount of working time before it sets, so these layers of mat and cloth had to be planned and cut out ahead of time. You'll note I've marked and cut dags and gussets in my layers of mat and cloth; Fiberglass mat and cloth can be bent over edges, but has its limits and doesn't stretch well at all, so it has to be cut where it wrapps around curved surfaces. You can almost think of it like paper; if paper would crinkle or have to be folded to conform to an edge, you need to cut it to create a dag or gusset. Indeed, if you haven't done this kind of work before, it can be a good idea to make paper templates of the fiberglass pieces you plan to use, and use them to plan and test your repair.

Fiberglass repair pieces need to be cut so they have large margins, extending 2-3 inches beyond the damaged area, and you can see where I've marked my gusset cuts to make it easier to line them up and wrap them into the inner curve of the wheel well when I lay them into the repair
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Old 10-06-2013, 01:13 AM   #3
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Mixing Fiberglass Resin.

Fiberglass resin is a plastic polyester that's chemically very similar to the polyester in some of the clothes we wear. Like epoxy, it comes in two parts, the resin and catalyst/hardener, which you mix together to get the resin to set. Unlike epoxy you don't mix them in equal parts, but use eight to twelve drops of catalyst (called MEKP, or MethylEthylKeytonePeroxide) per ounce of resin. Once the two are mixed, you have ten to twenty-five minutes of working time with the resin, less time on hot days or if you add too much catalyst, more time on cool days or if you don't add enough. Be sure to wear gloves when working with polyester resin!

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I bought my resin and catalyst at TAP plastics. They sell quart, gallon, and five gallon containers of resin, and sell the catalyst separately. A gallon of "bond coat" resin and matching catalyst runs about $55. I also have a handy pump that dispenses about one ounce of resin from the gallon container at a time, purchased from West Marine. You can buy replacement pumps at TAP for less, but, for some odd reason, the pumps TAP sells don't come with the right cap fitting to connect with their gallon polyester resin cans. So, I save the cap fitting from the West Marine setup and substitute in the TAP Plastics pump when I need a replacement.

In these pictures I've got twelve ounces of resin to which I'm adding 120 drops of MEKP catalyst before carefully mixing them for two minutes with a Popsicle stick. (The mat and cloth I laid out earlier took 14 ounces of resin to saturate and install, by the way.) Once the resin was mixed I started using a cheap "chip" paint brush ($7 for a box of 30 at Harbor Freight: you use a lot of them when you do fiberglass work) to dab resin onto the mat and fabric. The mat and fabric have to be "wet" all the way through before they can be applied. When the fabric has been appropriately wet through, it becomes translucent and amber in color with no white spots.

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When I said I dabbed the resin on, I really do mean dab and not use the paint brush to brush it in. During this step the paintbrush is mostly used to transfer the resin from mixing cup to fiberglass mat/cloth, then press it into the fabric. Stroking the resin as if you are painting tends to distort and warp the mat, and sometimes the brush can snag and distort the cloth's weave, so dab, don't paint!

I should also note that I'm laying up all 3-1/2 layers at once here. That's because I have had a lot of practice and know I can get all my layers up and into place before the resin starts to harden, and that I can apply one layer of wet fabric on top of another without disturbing my positioning of the lower layers.

If you haven't done fiberglassing before, I'd suggest doing all your layers in the same day (the fiberglass is stronger that way), but wait for one layer to set before working on the next.
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Old 10-06-2013, 01:25 AM   #4
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Getting the layers up.

Once all the layers you plan to apply have been wet through, it's time to put them on! Here you see me pre-wetting the area of my repair. In fiberglassing *everything* has to be pre-wet before you apply your layers.

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Here I'm laying up my first piece of mat (layer 1 in my layout) using my gloved hands to position the mat and the brush to coax and cajole the mat into the corners. Once the mat is in position, I then use the brush to press the mat into place and push any air bubbles out to the edges of the patch. (Yes, now you get to use the paint brush as a brush.) Air bubbles are unwelcome guests that weaken your patch, so you want to get rid of them.

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The half-layer (the pieces numbered with a "2" on my prep table) were installed in exactly the same way, centered over the tear in the wheel-well.

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Now the layer of glass cloth is going in. Same drill, but you can see how the markings on my dagged edges make it easier to line the cloth up with the edge of the wheel-well..

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And the top layer of mat. I didn't mention it earlier, but the top layer of mat is slightly bigger than any of the layers below. Often times repairs are done with alternating layers of mat, then cloth, then mat, and cloth then mat again -- always starting and ending with mat -- and each successive layer is slightly larger than the one below. That's an important thing to remember when your repair is in a visible area because it allows your patch to become thinner and less noticable at the edges, but in an unseen spot like this one, making each layer a little larger than the last is less important.

And, lest you think all of this was easy, you can see how this top layer is being a pill to get into place and flattened out.

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And the inside patch is finished! Now I get to wait an hour for the resin to set before moving to the outside.

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Old 10-06-2013, 01:33 AM   #5
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, but we don't need to wait an hour to see the pictures!
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Old 10-06-2013, 01:35 AM   #6
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The outer skin repair.

I started my repair on the outside of my trailer's wheel-well by using Bondo Glass to fill the crack and get rid of any potential air gap in my repair.

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Bondo Glass is the same stuff as Bondo Body Filler, but is green in color and has short fibers of fiberglass in it, making it much stronger stuff than the simple filler compound. It works something like a layer of fiberglass mat but, because the fiberglass fibers are thinner and shorter than those in fiberglass mat, not quite as strong.

Once the Bondo Glass had hardened, I rough-sanded the area smooth, and started sizing my fiberglass cloth patch.

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For this side of the repair, I'm using two layers of fiberglass cloth, one cut with the bias of the cloth, the other cut at a 45-degree angle to the first. Two layers of cloth are both thinner and stronger than a single layer of mat mat, which was important to me because and the area I gouged out with my angle grinder and sander was not very deep. As for cutting the patches so the weave went through at different angles, staggering the bias of the cloth improves the strength of the repair.

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The outside cloth got laid up the same way as the inside layers did. I wet the cloth, wet the area being repaired, then used my gloved hands and paint brush to press the two layers into place. The biggest difference when I did this part of the repair was the temperature of my shop, which was in the 55F range, well below the 65-80F working range of my resin. To compensate for these lower temperatures I used extra catalyst, 12 drops per ounce, and set up a halogen shop light to warm my working area while the patch cured.

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Once the resin holding the layers of cloth had cured, I topped the repair with Bondo Glass and sanded it smooth. (This picture was taken just after I started sanding the patch. I forgot to take a picture before I started sanding!)

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After sanding, I painted on a thin layer of polyester resin to seal my repair and give it a nice, smooth surface. If this patch had been a more visible area, I would have built up layers of Bondo Glass then Bondo Body Filler, sanding between each fill layer until my repair was smooth and the contours of my patch matched the contours of the trailer.

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Once the final topcoat of resin had fully cured, I lightly sanded the patch to get it ready for painting.
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