Fibreglassing experts, I have a question for you about repairs to my truck siding

nick disjunkt

Adventurer
Hi all,

I don’t know if there are any fibreglassing experts on here but I have a question which I was hoping someone could answer.

The body on my truck is made with a steel frame, onto which plywood siding is glued, and the edges and corners protected with aluminium capping. The siding is made of roughly 3/4inch plywood with a thin layer of GRP (fibreglass) laminated onto either side. On the back corner, water has got under the aluminium capping and has rotted a section of the siding behind the GRP. #

It is unnecessary and too much work to replace the whole panel, and so I plan to cut out and replace the rotten section, about 20 inch x 10 inch. Below is my rough plan of action, using a simple half lap joint and an overlapping layer of fibreglass.

CfVUrQg.jpg


I could buy a small piece of the same laminated plywood, but getting the join to be seamless will be awkward, and I think that my plan is stronger as the fibreglass will tie the new to the old properly.

My question regards how deep I should leave the rebate to glass up to. I have some 280g twill weave and some heavy 600g biaxial cloth, I plan to use one layer of each. If I rebate too far, I am happy to fill up to the level of the old plywood with resin and filler, but it would be a waste of material and would take longer. If the fibreglass ends up being proud I’ll have to sand it down which is no fun at all.

How thick do you think that the two layers of cloth will come out at? I can of course just do a tester piece but I thought it worth asking the question here first. I’m guessing 5mm looks about right,

Many thanks,

Nick
 

S2DM

Adventurer
Couple thoughts.

1) I wouldn't use epoxy glue then any poly resin. Epoxy soaks into wood very quickly, much more so than poly. Poly won't stick to epoxy so you may have an unbondable skin afterwards. I'd use a two part poly glue and be prepared to move fast, or just use a single part poly and wait a few days. Sika 252 or 3m 5200 both bond wood well.

2) I would try to get 3" of freshly bonded glass between the new panel and the old

3) after you glue your wood in but before you glass it, hot coat the wood. Catalyze a small batch of resin at 2% and coat the wood with it and let it soak in. Leave it alone for a few hours then lightly sand the surface for any resin excess. This saturates the wood and prevents the wood from pulling resin out of your bond.

4) a layer of 17oz biaxial cloth should be plenty strength wise, but make sure it has a chopped strand back. Wood isn't the best bonding surface for poly, epoxy works much better, but your other panel is likely poly, so you have a choice to make there (all epoxy for the new section, including epoxy resin, or stick with the poly with the following caveats). If using poly on wood, the bond will be much stronger by either using knytex/1708 which is a chopped strand backed cloth, or putting a layer of 3/4 oz chopped strand down before your biaxial. The chopped strand dissolves into almost a paste and yields much better adhesion (but very little skin strength).

5) finish your skin with either release fabric or surfacing agent. Poly doesn't fully cure in the air. The inside of the bond will cure, but you will have a sticky skin that will never cure and be a bear to sand off. You can paint it which will cure it, but it's better to either place a peice of nylon release fabric on the wetbond at the end and then roll on more resin at the end, or do one layer of resin and surfacing agent. The release fabric gets pulled when the bond has set up and leaves a cured sandable surface. The surfacing agent has styrene and wax in it. The wax rises to the top of the bottom and inhibits the air allowing it to cure.

6) use good resin. I'd get a gallon of vinyl ester resin. Much better than the cheap stuff and I would think the cost worthwhile if skinning plywood.

Feel free to pm me with any questions.


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Last edited:

S2DM

Adventurer
For reference, a piece of 17 oz 1708 mat is usually just a hair thicker than 1/16th an inch (sorry for the non metric there). It's easier to fill up with a body filler after than to sand glass down for sure. Using release fabric allows you to saturate the bond and then work the excess resin out with a roller which can make it thinner but just as strong if need be.


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S2DM

Adventurer
Rip stop nylon which can be bought at most fabric stores, works ok as a release fabric if need be. It's not the best but if you can't get release fabric (also know as peel ply) it's an option.


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S2DM

Adventurer
One other comment. It's not fun, but I'd sand your existing glass down around the new bond to 1/2 it's thickness. Taper it to nothing at the wood. Then let your new bond overlap the wood and 3" of the sanded glass/frp. Then smooth that afterwards. Butt joints won't have much strength in glass, you need to have fabric overlap.
 

nick disjunkt

Adventurer
Thanks for your detailed response!

I should have been clearer that I plan to use epoxy throughout. I have plenty of west systems on the shelf and I find it easy to work with.

I plan to bond the plywood to the steel frame where it touches with Sikaflex 221. I have a load of tubes on the shelf and it's undoubtedly similar to what the original body builder used. But I had planned to glue the lap joint with epoxy, sikaflex requires too much thickness for this kind of joint and I had planned to coat the whole repair piece in a couple of coats of epoxy anyway to guard against damage if water gets back in again.

I'll try to get the overlap at around 3 inches, it's larger than I had planned but it makes sense. But feathering in the new glass to old will be hard. These panels are laminated in a very thin layer of GRP, I guess not much more than 1/32. I'm not sure how the GRP skin is made but when cut there's no obvious fibres to provide a mechanical key.

The biaxial cloth I have has no CSM back but I've found it sticks to plywood well with epoxy resin. Maybe I'll just use the one layer of the biaxial and stick the rebate at about 1/8? I'll probably use epoxy with a lightweight filler to fair the durface, Its pretty easy to sand but sets harder than poly/talc body filler.
 

S2DM

Adventurer
The other possibility here. If the panel location is such that skin strength isnt imperitive, and you have enough surface area with the wood rebates to get a very well bonded surface, you could either

a) Laminate the wood with the epoxy before bonding it in place, make a very clean but joint, and then join that in with epoxy. If the frp is only for water proofing, that and a layer of paint should be sufficient.

b) epoxy your wood into place and then epoxy on an frp skin after you have everything as you want it. You could either buy this or make your own by laminating a piece of glass on a flat piece of melamine coated with a bit of release wax or mylar. You end up with a perfectly flat skin that can then be cut to fit exactly.

The epoxy really penetrates the wood. The yacht builder who acted as my mentor when I was starting my camper uses epoxy impregnated, painted wood on the exterior surface of boats, so it should be fine on a camper. Use a fine ply, finnish birch etc, with 7+ layers and do a good hotcoat on all sides before putting it into place. Can imagine that would degrade in the next 20 years.
 

nick disjunkt

Adventurer
I'm rather embarrassed about how long it has taken me to get round to this project. I was searching for the advice I was given when I originally asked the question and was shocked that more than 2 years had elapsed. In the meantime the West Systems had discoloured due to condensation but worked fine and the repair went exactly to plan.

This job has actually given me the boot up the ******** I needed to tackle a few other more minor repairs. I'm off the Scotland on Friday for a couple of weeks but will tackle them when I get back.

Here's some photos of how it went.

What the area looked like when I started. The thin GRP skin is the only thing left in that corner, the plywood behind it had rotten away completely.
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Getting to this point took nothing more than a gentle tug with my hand
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Here I am preparing the fence for routing the fibreglass rebate. Screwing router fence battens into good GRP seemed fairly brutal, but it was the only secure way of making a neat cutout, and I had to fair the repair piece anyway.
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Here the fibreglass rebate is made. I made a rebate of about 3mm
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Here the cutout and rebate for the repair panel has been made. The cutout was made using a plunge saw and guide rail.
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The repair piece was coated on the edges and back with two coats of epoxy resin to help it in the event that water gets back into this area. It was bonded in place using epoxy thickened with microfibres, and was temporarily held with screws. I later removed the screws and filled the holes with epoxy
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Epoxy thickened with microballoons was used to fill the gaps resulting from my poor measuring!
JXEssDP.jpg


The first coat of fibreglass is on and wet out thoroughly. I held the fibreglass in place with gaffa tape whilst I waited for the resin to gel
Y09InWp.jpg


The glass cloth cut easily with a sharp stanley knife once the resin had kicked
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After one layer of heavy biaxial cloth layer and one of finer twill weave, there was not enough depth left in some areas to apply another layer without it sticking proud of the surrounding wood.
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A slightly thickened resin mix was applied to fill the weave
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I had to put a decent layer of resin thickened with microballoons on, but I was still glad to have avoided sanding back fibreglass. I experimented with the random orbit sander to fair the area, but I was going through expensive discs too quickly. Using 40 grit paper on a hand sanding board was the best method. As long as I changed the paper frequently it munched through the filler in no time.
c5RaNUl.jpg


After sanding to 80 grit with the random orbit sander, I applied a couple of coats of International pre-kote primer. I must admit that I failed to mix the microballoons thoroughly enough into the epoxy resin, and was left with some pinholes in the surface. Using epoxy as a filler is a pain as it takes so long to cure, and so I used a tiny quantity of bondo between the primer layers to fill the pinholes
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I flatted the filler to 180 with the random orbit sander and refitted the corner pieces. I'm not sure why the sealant on these corner pieces failed in the first place, but the original coachbuilder was pretty sloppy and so it doesn't surprise me entirely. I was meticulous with cleaning both surfaces before applying a healthy quantity of Sikaflex 221. I applied one coat of Rustoleum Combicolor with a roller. When I have time I will flat this back with some wet and dry and apply a couple more coats. The colour match is good and with some careful polishing the repair should blend in well.
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Fender

New member
Nick,

Great repair! I appreciate your detailed instructions. If I remember correctly you had someone paint Jim for you. Was this the same paint and method (roller) that they used? Any big travel plans with Jim coming up?

Ben
 

nick disjunkt

Adventurer
Nick,

Great repair! I appreciate your detailed instructions. If I remember correctly you had someone paint Jim for you. Was this the same paint and method (roller) that they used? Any big travel plans with Jim coming up?

Ben

Thanks Ben, when I had the truck painted it was sprayed with a 2 part paint in a proper spray booth. With the huge number of scratches and marks on the paint after extensive use, I'm really not so concerned about the appearance now, and I'll be satisfied with the repair once its got a couple more coats and a few passes with the mop.

No plans for extended travels unfortunately. We're 2 days into a 2 week drive around the North of England and Scotland, which I'll post about in due course, but leaving my job and hitting the road seems unlikely to happen anytime soon.
 

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