Doing an exterior body repair. Any info on FRP panels, plywood, adhesives used

[fastline]

So I have a somewhat minor repair to make from tree limb damage to a newer rig with the "fiberglass" exterior. I am mostly looking for any videos or tech info one might have as to how they are constructed exactly, and materials used. I realize that can vary, but probably not much on the mass mfg rigs anyway.

From what I have determined so far is this thing they call "gel coat fiberglass" is really FRP or fiber reinforced plastic, and there is no "gel coat", just the original colored finish of the FRP.

I have worked with FRP years ago and some is very flexible, and some comes in sheet and pretty rigid, similar to formica. Seems to be bonded with a contact adhesive to a plywood substrate. Then that is glued to a stud wall which in my case appears to be 2x2 fir.

What I will have to do is obtain some FRP and build and glue in some patches. I would then gouge the joints and fill them with an FRP epoxy and sand smooth.

The glue that I see that stuck the exterior paneling to the studs is a white, somewhat flexible product, probably to purposely allow minor expansion/contraction.

If there is a technical repair section, feel free to move. I realize this is not something most people would get into, and apparently shops either because no one will touch it....

[theoldwizard]

Bare FRP will have a rough finish. You might even have some "print through" of the fibers.

The most common RV FRP is called Filon. Other brands available. It comes in a roll. Typically applied with contact cement, but use wat the manufacturer recommends.

Stay away from "construction grade" plywood, CDX. Very rough and at least one side will have void. Look ABX or ACX. At least one good face. Even though it use "exterior" rated glue, the wood itself will rot if exposed to water for a long time.

[theoldwizard]

Originally Posted by fastline

I realize this is not something most people would get into, and apparently shops either because no one will touch it....

The only place that I have heard that does this kind of repair is RV Roof Install. They have a YouTube channel. The specialize in TPO roofing, but have done a lot of Filon repairs.

[US1]

IMO.....I wouldn't just glue in patches, gouge the joints, and fill the gouges with epoxy. A good chance it'll just crack. Better off to taper grind the joint back so you can fill over the joint with glass bonding both the patch and existing. Granted the FG you are working with is thin compared to a boat, so you wont be able to grind very deep nor use a heavy mat. Id use chopped strand FG to fill the depression, sand smooth and use a filler as a final coat.

There is a guy who has very good/detailed YT videos of FG repairs.... boatworks today

[fastline]

Good input guys. I will look more at Filon. I realize, and willing to accept that this repair will not be invisible. I am simply not willing to go to extremes to get there. It will probably smoke another tree.

I intend to use cabinet grade plywood with exposure 1 rating (X). I usually use a birch veneer for side A but in this case, I might like to see a harder wood.

In any case, about the cracking, all areas affected will be solid blocked/reinforced. I'd love to add glass but just not feasible with the thickness and access. I have leaned on purpose built epoxy in the past, and some of that is insane good. I want to create a situation where those bonds are not having to deal with shear and tension though, via the reinforcements.

Probably my bigger and only real concern is is peeled off some of the Filon by peeling right through the plies of the plywood. Plywood never seems to glue back together right one that happens...lol The biggest issue here is getting that panel looking flat again. I am not worried about the structure. Studs are fully intact so once I block it up, shear strength will be fully restored.

So, you know, I am trying to get the "stack" thickness just right so it will look right again.

But as we say here, "do your best.....and caulk the rest"......

[Teamfoxy]

I've repaired previous RV's. The filing is glued to a 1/16th inch, 2 ply Luan and the plywood layers usually delaminate from each other. The plywood attaches to the framing, and floats over non-structural foam insulation. By the time you see bubles in the wall, there is often extensive mold inside the walls.

[theoldwizard]

Originally Posted by fastline

I intend to use cabinet grade plywood with exposure 1 rating (X). I usually use a birch veneer for side A but in this case, I might like to see a harder wood.

Most cabinet grade plywood does NOT use exterior glue. If you are going to cover it anyway, use BCX. Birch is not a really hard wood anyway.

Probably my bigger and only real concern is peeled off some of the Filon by peeling right through the plies of the plywood. Plywood never seems to glue back together right
In any case, about the cracking, all areas affected will be solid blocked/reinforced. The biggest issue here is getting that panel looking flat again. [/QUOTE]
Correct. If the plywood delaminates, replace it. Cut back to a stud. If the new plywood does not line up flush with the old, use a polyester filler (Bondo).


You will never be able to patch in Filon. Plus, you will have to buy a large piece, so use it.

[fastline]

Originally Posted by theoldwizard

Most cabinet grade plywood does NOT use exterior glue. If you are going to cover it anyway, use BCX. Birch is not a really hard wood anyway.

Probably my bigger and only real concern is peeled off some of the Filon by peeling right through the plies of the plywood. Plywood never seems to glue back together right
In any case, about the cracking, all areas affected will be solid blocked/reinforced. The biggest issue here is getting that panel looking flat again.

Correct. If the plywood delaminates, replace it. Cut back to a stud. If the new plywood does not line up flush with the old, use a polyester filler (Bondo).


You will never be able to patch in Filon. Plus, you will have to buy a large piece, so use it.[/QUOTE]

Yezzir, when I mention the birch, I just mean I have used that for "stuff" but certainly not RV repairs. I highly doubt someone will see a different in grade A vs B when bonded to FRP in this small area. I am more or less pissed how that plywood peels so easy. I was trying to minimize the repair area but wow. It's seriously like a shotgun victim on the table, like "where do we start?" These things explode like a grenade.

I do thing the word "bondo" has moved a bit over the years. A friend has 25yrs of dealer level experience and some of that bondo is no joke.

[Psneeld]

Throughout much of the world FRP (fiberglass reinforced plastic) is the term used for just about all glass reinforced plastic construction...not any type in particular.

I doubt you can get Filon that is very thin tapered enough to get a smooth joint with enough glass fibers between to be "crack proof" and not be a lump. Use an epoxy based filler versus a polyester based filler. I may be more flexible and if the plywood underneath doesn't flex too much it may work. The trick is to make or vee the joint just big enough to hold enough filler to hold but not so big as to show different than the Filon. Usually near impossible unless to take it to a natural eye break like trim, decal, etc.....

The alternative now that I have thought about it more....if the plywood is thick enough, then the joint between the Filon could be tapered and continued down into the plywood. Then filled either with glass fibers and epoxy or glass tape in layers with epoxy then sanded flush. Again... the width becomes the issue if the Filon shows texture and your joint does not. Only creativity will help you figure out what may help you there.

Can't speak to "cabinet grade ply" but most plywood these days is all made with waterproof (indoor/outdoor) glue. If worried, make up your own thickness ply by epoxying veneers together or adding a few to a thinner plywood.

[fastline]

Originally Posted by Psneeld

Throughout much of the world FRP (fiberglass reinforced plastic) is the term used for just about all glass reinforced plastic construction...not any type in particular.

I doubt you can get Filon that is very thin tapered enough to get a smooth joint with enough glass fibers between to be "crack proof" and not be a lump. Use an epoxy based filler versus a polyester based filler. I may be more flexible and if the plywood underneath doesn't flex too much it may work. The trick is to make or vee the joint just big enough to hold enough filler to hold but not so big as to show different than the Filon. Usually near impossible unless to take it to a natural eye break like trim, decal, etc.....

The alternative now that I have thought about it more....if the plywood is thick enough, then the joint between the Filon could be tapered and continued down into the plywood. Then filled either with glass fibers and epoxy or glass tape in layers with epoxy then sanded flush. Again... the width becomes the issue if the Filon shows texture and your joint does not. Only creativity will help you figure out what may help you there.

Can't speak to "cabinet grade ply" but most plywood these days is all made with waterproof (indoor/outdoor) glue. If worried, make up your own thickness ply by epoxying veneers together or adding a few to a thinner plywood.

All in all, I am willing to accept the fact that this repair will be noticeable if looking for it. I am not going to rebuild the camper over it. I did a paint repair on my show truck many years ago where I best friend/pro painter said I would need to remove the part and repaint. I ended up filling the chip with base with an air brush, then feathering/building the clear over it, then wet sand and burn in. Nearly impossible to find.


Anyway, what I need to do now is source materials. Where can a guy get a piece of "Filon"? I also need to find an adhesive. This is one of those things where that glue is everything. If I choose poorly, this will be a disaster.

[US1]

You can use the epoxy to bond filón to your plywood. You’ll separate the plywood before the bond would fail. I always mix cabisol with the epoxy for a foolproof bond. It also fills any voids.

[Bamaman]

The first week my new fiberglass sided fifth wheel, I had an accident and punctured the siding and impaled my refrigerator and cabinetry.

It took Grand Design about 5 months to replace the cabinetry.

The fiberglass siding was repaired by gluing thin plywood to the back side and finishing it off with fiberglass patch and Bondo. Grand Design gave the paint color mixture to the body shop. After the paint was applied, they had to use wadded up plastic wrap to faux finish the wet paint--to get the sheen to match. 6 years later, the paint and repair is holding up okay.

[Dav L]

post a picture of what you are trying to fix. But if it is FRP / Filon etc, then get fiberglass cloth and resin and fix it.

[JoeJunior]

RV body shops patch the fiberglass siding from behind and they often will glue a new patch of the same siding.

The cracks will be filled with plain old Bondo body filler and then it'll be sanded smooth.

The siding will have to be spot painted and then clear coated. The final coats on my Grand Design was faux finished to match the texture of the fiberglass which is actually a little rough. The painter used a ball of plastic wrap and he dabbed the final coats. I'm just glad that Grand Design furnishes the paint codes on their finishes. Most RV's colors are not uniform and the painters have to spend a great deal of time matching the paint colors by trial and error.

RV painters have to be extremely talented, and great painters are very hard to find. To repaint a whole 40' motor home would be extremely expensive in just paint and supplies alone. And who knows how long it would take to paint a multi colored big RV? And at $150 an hour labor, I'm surprised RV insurance is as cheap as it is.