Some Basic Fiberglass questions

[Dan H]

I have a question or two with regards to fiberglass repair.
When repairing a hole in the laminate, after the area has been correctly beveled and prepared, fiberglass pieces are cut to fill the area. Each piece is a little larger to fill the tapered void.
Jamestown Distributors has a video on You Tubeand says to place the largest piece of fiberglass into the bevel first and then the successively smaller ones towards the outside of the hull.
Don Caseysays to place the smaller ones in the hole and work out with successively larger pieces of fiberglass.
West Systems say to place the largest piece in first.
It seems that the fiberglass works under tensile strength and by laying the largest patch in the hole first, the fibers will curve to match the bevel. This bowing of the fibers reduces the tensile effectiveness because the fibers have to straiten out before they come under tensile load.
I would think that you would want the fibers as flat and strait as possible, as in the way Don Casey says to do it.
I just don?t see how the successive laminates add strength to the repair if they are all smaller than the first layer. If the bond of the first and largest piece fails, all the bonds fail. Which way is correct here?
Second,
I have been told that epoxy is never used with CSM. Ok I agree, but what about the biaxial stitch mats? The 1708?s that have the mat sewn to the biaxial? A person on one of the boat building web sites told me that even the mat sewn to the biaxial should never be used with epoxy. He claims it adds too much weight and is a waste of epoxy.
Biaxial isn?t used with polyester resins so why would they make stitch mat for use with epoxy if you should never use it with epoxy?
The West Systems guys told me that biaxial / mat used with epoxy, mat side up provides a great moisture barrier. They think it is a great idea. They even sell the stuff. This person tells me that they want me to use it because it sells more epoxy. I honestly think West Systems is above that. What is biaxial stitch mat used for?

[Ryan]

Dan

I have always used the largest piece first and add the smaller pieces as you go. You will no doubt get some great answers and reasoning from the members here, but at the very least, this method prevents you from sanding away as much glass when you fair the repair. Keep in mind that if the layers are laid up while the epoxy is still green, then the entire repair is chemically bonded to itself. So regardless of your method, the only mechanical bond is the new laminate to the old. In other words, the new laminate has one big bond to the repair area, not several small bonds (IF the laminate is finished while the epoxy is still green). If it cures between layers, that's another story.

I used five layers of biaxial with stitch mat and epoxy to laminate a new deck over balsa on a 25-foot boat. It built up the deck thickness very easily and turned out great.

[MQMurphy]

I'd like to hear a few more replies to this, too. All due respect, Ryan, but I've seen the contradictory directions in a couple of places as Dan has.
Smaller up to larger was the recommendation in Allan Vaitses' book, if I recall correctly. Anyone else want to chime in here?

[Hirilondë]

Larger to smaller when sanding for the very reason Ryan states (sanding through layers reduces the structural integrity of the repair). The opposite when you want to smooth over the repair with the cloth and resin The largest one covers all the frilly ends of previous layers.

[LazyGuy]

If you look at it as an exaggeration it may make sense. For starting with the small patch (the Casey method) picture the multi layer patch as a series of steps like a pyramid and turn it upside down against the bevel. There is plenty of space for solid epoxy and small areas where the cloth is in contact with the bevel. We know that solid epoxy does not make a good patch plus we are not maximizing the contact area for the cloth. For the big patch first (the West method) you are always maximizing the contact area and you end up sanding away the solid epoxy area. I have seen the Jamestown video and the only change I make is that I lay the first piece of glass, wet it out and squeegee the daylights out of it to ensure it is the best bond. The remaining patches I drop in as a single piece because all the new glass will cure as one piece.

I understand that they are making the mat differently now so that it will work with epoxy but do not quote me, I have not tried it myself. In fact, I hate using mat period. A friend of mine that builds plywood boats then covers them in fiberglass will use nothing but the Biaxial Mat with epoxy as a first layer for a number of areas such as where the deck meets the hull, on either side of the keel on the inside of the boat and at the transom.

[catamount]

Larger patch to smaller gives the most continuous contact between the fibers in your patch and the original material being patched. As suggested above, it does also prevent you from sanding through those continuous fibers when fairing the patch.

My own approach is usually to go larger to smaller and then back to large again! More layers of lighter cloth is supposed to be stronger than fewer layers of heavier cloth, although it is more work...

As for mat and epoxy, the story I got was that mat is made with a binder that holds the fibers together, but that in the typical mat you pick up just about everywhere the binder is chemically incompatible with epoxy, resulting in a weaker laminate. But, obviously one can buy mat that is compatible with epoxy, and if that is what you are using there shouldn't be any problem.

That said, I generally prefer to work with woven or stiched materials rather than mat, because the chopped fibers in mat make for a god-awful mess!

Regards,

Tim A.

[Ceasar Choppy]

I'll add my hat to the larger piece first ring for the same reasons as mentioned above.

If you want to overkill it a bit, bevel BOTH sides of the hole in the hull and do the same to both the inside and out. That plug won't be going anywhere because it has a chemical bond holding it together! But again, that is usually overkill.

CSM is a pain because it sucks up so much epoxy, and Catamount is correct about the binders sometimes being incompatible. But I've worked with the mat stitched to the biaxial and love it because it is compatible and adds thickness. I find the most important step to all of this though, is making sure you properly wet-out the FRP as a first step.

[Tim]

The mat on the back of biaxial material is lightly stitched in place and doesn't use the bulky, stiff binders that you find in plain stand-alone chopped-strand mat (CSM). So the binder dissolution issue doesn't come into play.

Mat on the back of the material helps stabilize the cloth when wet out (and before), so that it doesn't stretch and become distorted. It also adds bulk, which is often a positive factor, though not necessarily always. The mat also encourages good bonding between multiple layers of the material.

Calling material with light mat on the back (such as many biax materials) "stitch mat" is a common slang, but frankly it's misleading since it tends to make one believe that the mat is main component. It's not: it's only there for the reasons above, and is secondary to the woven cloth fibers. "Stitch mat" is not mat: it's biaxial cloth with chopped strands stitched to the back.

As to the largest or smallest piece first, here's how I approach it. Both methods under consideration are correct, but each is more correct for one specific type of fiberglass operation than another.

1. For patching a hole, start with the largest and move out to smallest. This maximizes the contact area of the repair with the ground bevel around the repair, and minimizes the amount of structural material that you grind away when you fair the patch.

2. For tabbing and similar operations, or rough patches where you aren't grinding a bonding taper, start with the smallest piece and work up to the largest. This spreads the bonding area over the widest possible area, and the overlapped layers in this case add to the strength.

This person tells me that they want me to use it because it sells more epoxy. I honestly think West Systems is above that.

They are above that. Anyone with a brain understands that.

There are a lot of valid reasons why a person might choose another epoxy brand, but this vast West System Conspiracy theory that some people like to spread is simply unnecessary, unfounded, and just plain silly.