Questions about filling in a couple 15" wide holes with FRP sheets please

I need to fill in two 15" x 15" former roof vent openings in the roof of a 1982 Toyota Sunrader motorhome shell that was constructed with a chopper gun and polyester resin. The roof/ceiling is 1/8" to 3/16" thick and I'm going to glass in the holes with a sheet of 1/8" thick FRP. The joints need to be strong enough to walk on like it was before (after I shore up the interior ceiling with new wood or metal rafters).


Is this the best way to proceed...

1) Cut the FRP sheets so they are about 3/16" narrower than each hole they will be filling. So if the hole is 15" x 15" than the FRP sheet would be approx 14 5/8" x 14 5/8" to allow lots of room for the epoxy and glass on each side of the hole to "mate" with each other.

2) Glass the panels in with epoxy resin and 3" wide strips of 6 oz E cloth on both sides (interior and exterior) of the panel.


Questions please...

a) Is epoxy overkill for this application and would good ol polyester be okay?

b) These little FRP sheets are light so I was thinking of holding the FRP sheet in place from the interior side with some kind of tape. Is there a tape or something that epoxy won't adhere to? Polyester resin won't adhere to wax paper but what about epoxy? How would you hold the FRP panel in place from the interior so it doesn't fall through while I glass it from above (from the exterior) and while it cures?

c) Lastly, and sort of unrelated... What material would you suggest I use for "rafters" to stiffen the ceiling? Wood or metal? I will glass them in with epoxy. They used crappy particle board 1" x 2"s before but I want something stiffer. I was looking at this 1 5/8" tall "Z" shaped metal they used in a box van recently. What do you think?

Any ideas will be greatly appreciated!

Thank you

Steve

 

Either resin system will work. Epoxy will bond better and be ultimately stronger, though you may not need this.

The best way would be to feather the edges of the openings back, so they are at least half the thickness of the original laminate. Ideally, you'd do this from both sides, grinding back say 3" or so all around. Next, place a piece of light (1/4") plywood on one side, wedge or weighted in place, to serve as a temporary backer. Cover the plywood with plastic sheeting or packaging tape, so the goo can't stick to it. Next wet out the feathered areas with goo and lay in a piece of fabric just a smidge short of the inside diameter of the hole. Wet this out with resin, then apply more fabric, with slightly larger diameters, until you're just a wee bit shy of being flush with the surface and the overlaps have climbed up onto the tapered, feathered flanges, ground around the hole. Let this cure. Once hard, remove your piece of plywood and do similar to the opposite side, making sure you "tooth" up the new 'glass before apply the next round of 'glass work. The freshly applied 'glass will now serve as the backer, so toss the piece of plywood.

If using polyester or vinylester, use a "combimat" product, which is a cloth with a mat lightly stitched to it. Lay each layer in with mat to cloth contact. If using epoxy, you don't need the mat, so a straight cloth or knitted fabric (biax) will do.

If you work neat and the plywood piece a good tight fit, you could cast at least one side, with a near perfect finish, but this does require some experience with goo and fabrics. This is why I suggest stopping just short of being flush, so there's room for a filler to level up the surfaces. I don't like using wax paper as the wax can come off during the exotherm part of the cure, contaminating the bond surface. I prefer plastic sheeting, Mylar or plain old, plastic box tape.

Your metal reinforcement can work, though bond issues will potentially causes problems, because of expansion dissimilarities. I'd use another 1x2, though if you need some real strength, make them from hardwood. If you just need some additional support, rip the edge off a 2x4 (.75"x1.5"), which will be pine or fir. A good material for this is the untreated Douglas fir available at Lowe's/Depot, sold as decking. It's 4x4ths stock (7/8" - 15/16") and usually 5.5" wide. Just rip it to the width you need (1.5" or so). These pieces are usually very straight grain and strong for their weight. Use thickened epoxy to bond them in place, with a very small fillet along the edges to transition from the new "beam" to the overhead. Of course, the wood needs to be precoated, with 2 coats of straight goo, before getting bonded in place. lightly radius the corners so it looks nice and the goo doesn't have a hard edge to release from.

 

Thanks for all the great info PAR! I'm pretty good at glassing cuz I used to make all my own surfboards but its been a while so your answers helped a lot to refresh my memory.

I will for sure check out that fir decking material before I proceed. The rafters only need to extend 6.5' across so last night I experimented with a piece of 3/4" x 1.5" poplar with another piece glued to the bottom edge so it formed an 'L'. I was surprised how stiff it was. I'm not sure what material I'll use but I'm thinking of doing something similar but I'll plane an arc or crown into the top edge that glues against the interior ceiling so as to minimize sag in the ceiling.

A couple more questions please...

-I always thought epoxy adhered to anything so are you saying that epoxy will not adhere to clear packing tape or plastic sheeting? Meaning that I should be able to easily peel it away from the epoxy without ripping?

-Have you ever used the clear liquid catalyst used with good polyester resin to cure Bondo?

Thanks again.

 

Epoxy doesn't tick well to several things and polypropylene, PVC, Mylar, polyethylene and numerous others make the most common materials list. Plastic packaging tape is usually PVC or polypropylene, both work well as a release film. I have tools covered with this stuff, to keep goo from making a mess of them. When using clear plastic sheeting, it can pucker under clamps or during the exothermic portion of the cure, so I'll staple, spray adhesive or other wise insure it stays where I want it. Mylar is much better in this regard as it's stiffer and usually thicker, but it's costly. I've also used "plexiglass" (acrylic) and polycarbonate sheets to make dead bang perfect smooth surfaces for table tops, work benches, etc. Once the goo is cured, you peel from a corner and it'll release, leaving a mirror image of the surface you're removing. This is a handy feature when doing textures too, as you can buy textured PVC panels at the big box store and use these to cast seamless textured panels. Just peel the PVC off when it's cured.


I've very little use for Bondo and polyester resin systems, but I'm reasonably confident (knowing it's chemistry) the catalyst used in Bondo, is nearly the same as typical polyester resins, with the exception it has a pigment (could be a die) to give you an idea it's mixed properly.

 

Hey thats a good idea to cover tools with clear packing tape.

I hope epoxy will adhere to PVC because I might be using PVC pipe or parts of PVC pipe for areas on the fiberglass shell. I'll do some experimentation first though.

I read this at the West System site...

"Typically, epoxies don’t adhere well to many plastics. But, with WEST SYSTEM G/flex® Epoxy and proper preparation, you can achieve excellent adhesion to most plastics. Sand ABS, PVC and polycarbonate plastics with 80-grit sandpaper to provide texture for improved adhesion. Some plastics like HDPE and LDPE (high-density and low-density polyethylene) benefit from flame treating. First wipe the bonding surface with a solvent to remove contamination and dry with a clean paper towel. Pass the flame of a propane torch across the surface quickly. Allow the flame to touch the surface, but keep it moving—about 12 to 16 inches per second. The flame oxidizes the surface and dramatically improves adhesion with adhesives and coatings applied over it."

Have you tried that technique with PVC where you pass a blow torch over it?

 

Yes, this is called caramlization and it works to a degree, but you still don't get a real good structural bond. PVC in the usual forms (pipe, extruded shapes, etc.) usually isn't very strong, especially for it's weight, so I'd reconsider it's use, if it's structural.

 

Do you know how well epoxy bonds to ABS plastic? Is it stronger than its bond with PVC?

 

All the commonly available "pipe" plastics can benefit from caramlization to improve the bond, but there are things to consider with these materials. First, they are pipe, not tubing, which is an important distinction. Tubing is structural, pipe just carries stuff, like water or electrical wires, but isn't typically considered structural. So, the choice of a pipe isn't a good idea in structural applications and then the bond issue crops up. To give you an idea of how poor the bond is on uncaramlized plastic surfaces, I use PVC sheet goods from the local big box store, to cast textures into epoxy coated surfaces. It does stick, slightly, but is easily peeled back, revealing the cast in place mirror image of the texture in the cured epoxy.

PVC and ABS can be used for wire and plumbing "chases" or other none structural applications, but I'd be very leery of it holding any load. It also has a huge difference in environmental expansion and contraction, compared to most other materials, including epoxy. This is so much so that attaching PVC and similar plastics to other substrates requires elongated fastener slots and light fastener pressures, so the material can move with temperature changes.

 

What PAR said goes double for ABS. I really moves about, expands and contracts with temperature changes.

 

A) Regarding your instructions below for gluing a wood "rafter" to the ceiling of an 1/8" to 3/16" thick polyester resin surface... You don't think a layer or two of 5" wide fiberglass tape epoxied to the roof and up each side of the rafter after the fillet would be required to make the roof strong enough to walk on?

B) What kind of filler should I use for the fillet considering I'll be working overhead? I can buy cabosil close by at Mitches Surf Shop but not sure cabosil is strong enough. And should the filler I use for the fillet be the same stuff I use to thicken the resin for the bond where the wood meets the ceiling? I could use chopped strands of fiberglass for the fillet but I don't think that stuff will stay in place in the time it takes for the epoxy resin to harden.

C) Regarding the kind of wood to use for the rafters... I did a flex test with pine, birch and oak 1" x 3"s set on edge and the birch was as stiff or maybe stiffer than the oak. Do you know how stiff fir is compared to birch?

D) Is this the 1 x 6 fir at Home Depot you were referring to? You said it was sold as decking but a search at Home Depot for "douglas fir decking" only brings up the usual 2X material. http://www.homedepot.com/p/1-x-6-x-8-Standard-Douglas-Fir-Hem-Board-174152/202059519#.Uf_sVePXivI

Thanks again!

 

No, a few layers of tape on the overhead will not do much. It'll be as flexible as a ****** outside a GM plant on payday. You'll need a "localized stiffener" (beam) which can be one of several materials (wood, metal).

Cab-o-sil is strong enough, but you'll want to use a high percentage of milled fibers first, using the cab-o-sil as a viscosity control, more so then a bulking agent. Chopped mat is useless int his application and it has no real strength. You'll want cloth or biax as the fabric, no mat. Mix the milled fibers and cab-o-sil to a peanut butter consistency, so it will not sag. If you pull a putty knife through it and the little lumps fall back down as the knife passes through, it's not thick enough so add more cab-o-sil.

All three species will work. With the denser species, you can decrease the beam dimensions and/or on center spacing.

It depends on where in the USA you are, but in the decking section, which is in the isle where thy keep the hardwoods, not the pressure treated stuff, you'll find straight grained cedar and Douglas fir, usually 4/4's x 6" decking stock. It'll have a bull nose on each long edge, though sometimes it's tongue and grooved. Web sites are standardized and you physically have to go to the store and look at their stock.

 

I think you misunderstood my question. My intentions were never to use fiberglass tape alone. I was asking if you think I should use fiberglass tape to glass the edges of the WOOD to the ceiling after I do the fillet?

Would you consider these 1/4" chopped fiberglass strands http://www.fiberglasssource.com/sunshop/index.php?l=product_detail&p=940 with as "milled fibers" I could mix with the cabosil for the fillets?

And would you use the same thick fillet mixture for the edge of the wood that is glued to the ceiling...or should that mixture be a little thinner than the fillet mixture?

And IF that mixture should be thinner than the fillet mixture, would you use both milled fibers and cabosil for that too, or is cabosil only good enough?

Thanks again.

 

Sorry about the confusion.

If you bond the beams in place with a bed of thickened epoxy and a generous "structural" fillet on each edge, you might be okay. Tabbing (tape/fabric), is the traditional fashion of attaching these elements. It difficult to tell without pictures and a better idea of what you're attempting. I'll suspect you'll be fine, though 1/4" fibers are way too big to make anything close to a smooth fillet. Milled fibers are much smaller, usually 10 to 20 times so. A 1/32" milled fiber makes a rough, but workable fillet, if you add a good bit of talc and cab-o-sil.

To bond the beams, the area they bond too, needs to be ground clean of gelcoat and wet out all contact surfaces with straight epoxy. Next goes a thickened mixture, which could be a little thinner than the fillet mix, but why bother, because you'll do these at the same time, most likely. As you wedge the beams in place, with just enough pressure to hold them there (no more), the goo will ooze out along the edges, which you can use to start the fillets. Apply a little more thickened goo and complete the fillets in a uniform radius, the full length.

 

I started this thread with questions about filling only 2 openings in the roof of my Toyota Sunrader motorhome (with fiberglass body) but I counted them all yesterday and I'm actually filling in 21 openings! I haven't started on the wood rafters for the ceiling yet but that will come next after I close up all the holes.

I have some questions please...

Set up for question 1: In the first two photos you can see a few of the openings I'm glassing in solid. I've been glassing in the openings with 1/8" thick fiberglass panels (which is approximately the same thickness as the fiberglass motorhome body) using polyester resin and about 4 layers of 6 oz E cloth on the interior. I filled the gaps on the exterior with a thick mixture of polyester resin, 1/64" milled fibers and cabosil. When I'm done all that stuff will be primed and the entire motorhome painted white. My concern is that I've seen body work where big dents were filled thick with Bondo and later the Bondo cracked underneath the paint. I'm hoping the filler mixture I used on the exterior of polyester resin, 1/64" milled fibers and cabosil won't crack later because most of the stuff used to thicken the resin was milled fibers.

QUESTION 1: Should I sand a concavity around each opening where the exterior filler is and lay some cloth in there to avoid possible cracking (like thick Bondo is prone to do) or do you think I'll be safe from cracking because most of the stuff used for thickening was milled fibers (and the body will be painted white)?


Set up for question 2: In photos 1, 3, and 4 you can see where I removed an aluminum molding that was used to hide the seam where they joined the top half of the Sunrader to the bottom half. I took it off because it was bent and funky looking and was attached with a million rusted screws. The problem is that now I'm left with an extremely rough and ugly seam. What I want to do is run a nice smooth straight fillet of thickened resin along the bottom of that over-lapped joint.

QUESTION 2: Do you have any suggestions for how to make a clean smooth fillet when the joint I'll be running my stick along with the thickened fillet goop isn't a smooth straight line at all?

Thanks again!

 

Yes, you should "feather the edges" back, making a concave dent around each seam. This offers a place for the thickened goo to live, without it being above the surface. You should also include some fabric as well, so the seam will not "print through" from below.

As to a fair line, the usual method is to fill and reinforce, so the repair is sound and treat the cosmetic portion separately. After applying the fairing compound, use a piece of tape to suggest the line you want and sand only to it from one side of the seam. Place another piece of tape on the other side, and work to this piece. You're sneaking up on the line so to speak, using the tape as a guide to keep you from sanding "over the top" of the line, you're trying to create.