Structural Crack

[kyle minor]

Today i was looking at all the gel coat spots I need to fill when i discovered a serious crack at the rear of my '85 Burro. It is located exactly opposite of the spare tire, so I went around back to see whats up. It appears the previous owner torqued the carp out of the bolts, compressing the wheel into the fiberglass shell so deeply that it cracked the shell at the seam where the 2 molds are joined. After i removed the wheel I could clearly see light through the crack and realized this is a serious structural concern. See pic below for a better idea.

How do I repair this? I've heard that you can't apply polyester resin on top of cured fiberglass and the only way to adhere new glass is to use epoxy resin. Is this true? I have done a fair amount of fibrglass work years ago making large scale public sculpture but am new to the repair scene. I even have a decent amount and variety of fiberglass and carbonfiber fabric laying around. I want to know how to prep the surface and what material to use to bond the new and old surfaces to achieve a structural bond. Any help/direction/pointers are greatly appreciated.

Thanks!

[Raya]

Hi Kyle,

From what I can see, and what I know of how a similar trailer is put together (U-haul), that looks like a relatively straightforward repair.

The Burro was made with polyester resin. As you probably remember from your past fiberglassing, there are two types of bonds: Chemical and secondary (or physical).

You can use polyester resin, but it does not have very good secondary bonding characteristics, as compared to epoxy. Now, it would probably be fine for the repair you have there; but, on the other hand, why not use epoxy? I actually prefer it because it does not have the (to me) noxious smell that poly- and vinylester have. It is slightly more expensive, but in the amounts you will be using it will be negligable. And the secondary bonding with epoxy is superior. (You can no longer get a chemical bond because that has to be made within hours of the original fiberglass being laid.)

I would use a biaxial reinforcement (that means the fibers run on the diagonal and they all provide strength; whereas a 90º type cloth would have half the fibers running parallel to the crack and not really helping). A "tape" style will make for nice, neat edges, but you could cut repair strips out of a larger piece, too.

"biaxmat" is a reinforcement that has a layer of biaxial cloth stitched to a layer of mat, and it would work well. Now, like with the resin, you could get away with something else, as it's not a major repair in terms of strength.

So I would proceed roughly as follows:

1) De-wax with something like Interlux 202 Solvent wash
2) Tape/plastic off surrounding areas
3) Sand area to be repaired (for smoothness and tooth).
4) See if it smoothish or if there is a "rut" that would be hard to fiberglass (if so fill it first with thickened epoxy).
5) Cut a couple of layers of reinforcement (cloth) to fit; stagger the size slightly (you may only need one layer, depending on weight of cloth)
6) Wipe surface with acetone or denatured alcohol
7) Wet out surface with "neat" epoxy
8) Wet out reinforcements with neat epoxy (to the side, on plastic or in a dishpan, etc.)
9) Apply reinforcments to prepared area
10) Brush/squeegee to remove air bubbles and excess resin
11) Leave to cure (tape cloth in place if it wants to fall down).

Since that area (I think) gets covered by either carpet or a bench or something you only need to make sure it is relatively smooth, so cosmetically it is not too demanding.

Schools of thought vary on whether to put the larger or smaller piece on first (you probably don't even need more than one layer, really), but either would work fine here since you are not going to be sanding/fairing.

This will now be the strongest area on the trailer

I would just aim for covering out to the white section, but not overlap it.

You probably remember this, but it's important to wear protective clothing, gloves, eye protection, and a respirator.

Raya

[kyle minor]

WOW! Now that's the kind of response I was hoping for but not expecting. Thanks!

You spelled everything out very clearly but I have a few questions. Can I clean the area with acetone initially to remove wax and carpet glue? How well do I need to sand the area in question? My thinking was that the surface is very rough with considerable "tooth" for the bond to hold onto and that maybe it's a good idea to just clean it thoroughly before wetting the surface. Does sanding reveal a better surface to bond to chemically?

I have a fair amount of heavy strand, 90 degree weave fiber that I will cut at a 45 degree angle to achieve the type of strength that you describe. If I remove the bolts that the wheel hangs on, should I use a different material to fill the hole or just chop up small pieces to fill and tape the back side to prevent blow out?

On a similar topic, I wanted to add a drop leaf table to the front area as a small dinette and keep the back bed permanent. What would be a good way to attach the piano hinge and cantilever support to the egg wall? I was thinking about shaping pieces of wood or marine ply to follow the curve of the wall and either epoxy them or possibly add glass as well to bond to the wall. I could then screww my hinge and support to the wood. Any recommendations? Initally my thought was to use aluminum but i quickly abandoned the idea after I read about bonding issues. Any thoughts on tis?

Thanks!