|Topic Review (Newest First)|
|12-08-2007 02:15 PM|
I asked Don Casey the question directly and got a very comprehensive logical reply. The answer is that biggest first is best but lots of people have done it the other way for a long time and it has worked OK.
As a point of reference I just watched the http://www.bennettmarine.com/store/s....php?cat_id=59 Bennett Marine video and they show the expert using a patch with all layers the same size “large” and just slapping it in. It left a lump around the outside edge that he just ground off.
So it looks like we now have Don Casey, West Systems and Bennett all saying about the same thing.
Also this minor flap as shown how forgiving fiberglass is as a material for repair.
>>> Don Casey Response
Your original question:
Assuming a 3" hole in a 1/4" fiberglass skin ground to a 9" depression a West video shows the 8.5" piece of repair glass being the first piece laid down while your books show the 4" piece as first and 8.5" as last.
Do you think it matters? Would one way be better in some circumstances than others?
Also some people say patches on a glass hull can use epoxy while others say it is better to use the original resin to match the shrinkage and flex of the original material and not create a hard spot. Who is correct?
Our expert's answer:
When filling a depression the intuitive order is small to large but the problem here is that we are trying to replace cut-away or ground-away fabric and the new fabric, except for the bottom piece, will only attach to the original material at the perimeter—in effect a butt joint. Laying the largest piece first maximizes the surface area of the secondary bond. After that all subsequent laminates bond to this first piece and each other on a molecular level but applying them in a large-to-small order still maximizes the mating surfaces.
In addition, when using polyester or vinylester resin, the first piece must be mat. If it is the largest piece it gives you the required mat base for all subsequent laminates. If it is the smallest piece, even if it is mat, the next layer, which will be cloth, will not have mat between it and the existing laminate beyond the perimeter of the underlying piece. This will make for a weaker bond.
If you are using epoxy resin, the order of the laminates makes less difference to the strength of the repair. Small first tends to yield a neater appearance but the void created where the larger layer overlaps the edge of the one beneath can create a void that lets the thinner epoxy resin drain through unless you thicken it. A large-to-small order avoids this. Also the relatively recent development of epoxy-compatible mat makes a large-to-small schedule using mat between the cloth layers a possibility.
Bottom line is that despite counsel to the contrary in a number of books with my name on them, large to small is usually better unless you have a specific reason to do otherwise. A new edition of This Old Boat out late next year will reflect this change in thinking and technology.
As for which resin to use, I think the better adhesion of epoxy trumps all concerns about differing flex characteristics. Here again, however, you are right to question the “experts.” The fact is that some fairly recent testing show a secondary epoxy bond just 15 percent stronger than polyester. If you were thinking it was more than that, this might influence your choice of resins based on either cost or consistency with the original lay-up.
>>> >>> End Don Casey Response
|11-15-2007 10:04 AM|
Small to Big
Originally Posted by haffiman37 View Post
When filling the hole, feather the edges on the outside. The feather edges will provide some surface for a "bite". Your goal then is to get the outermost layer in the hole (the side towards the water or outside) as filled and dense with fiber or filler as possible. If it is resin rich, it will end up cracking.
Agree about the material matching as best as possible.
|11-15-2007 09:40 AM|
we usually go small first, that way your largest piece is usually just mat, there basically for the fairing,,thats why we go from small to big, and because if you start from inside where the strongest glass is, you only take out a little of what matters, leaving the cosmetic (largest) outside piece last. this i think gonna turn into the " chicken or the egg" threads haha
as for #2 heh,,you got opinion
|11-15-2007 09:33 AM|
|SimonV||The lager piece goes in first.|
|11-15-2007 09:16 AM|
So what is the bottom line here?
There seems to be two basic disagreements.
1. If you have a 4" hole in a 1/4" thick hull tapered out to a 10" depression do you put in a 5" piece first (1/2" overlap) or do you put the the 10" piece first and let it belly to the bottom of the hole?
2. Are you better off using epoxy for extra strength or poly to match hardness and shrinkage of hull?
If I got the questions right I may be able to find someone with an engineering answer.
It looks like any combination will work as we have two experienced people with different takes. The bottom line may be that it dosn't matter much. Or there may be certain conditions that favor one approach over another. I just want to make sure I have the question right before I take it to the engineers.
|11-12-2007 11:12 AM|
i cant say what they did,, i wasnt helping, but i do know they knew a very lot bout epoxy, and worked with it for years. as of me i've used epoxy maybe enough times all together to stick 10 boards together ( short boards) , and it was them same oldies that told me how to repair it and why it happened, then handed me a 9 inch wildcat and said cut-em out.lol the perks of seniority
so when it comes to epoxy im a idiot,,and will be learning forever,, but i do know what i've had to deal with and how the "teachers" taught me to do it so i didnt have to do it again lol
|11-12-2007 09:46 AM|
While it may not have been mixed incorrectly each time, the person, if unfamiliar with epoxy work, may have used the wrong type of hardener or done something else, like mix epoxy in the wrong type of container, and contaminated the epoxy each time. In one case, down at my marina, a guy was using waxed paper cups to mix epoxy... with somewhat unpredictable results.
Originally Posted by the1much View Post
|11-12-2007 09:05 AM|
|the1much||and i fergots bout the way the epoxy was mixed,,, all 3 guys have been doing glass work for over 30 years each,, they tried telling the owner not to use epoxy,, but he got it in his head that it was stronger so it had to be better,, and i cant imagine they did ALL 8 port lights in 1 batch of resin,, and i cant imagine they could mix it wrong all 8 times. and i know them slow old dudes didnt do it on same day.|
|11-12-2007 08:56 AM|
Working from Inside
If the damage area is small and above the waterline, make the repair from inside the hull, if possible. You are going to bevel the edge of the hole with a 12-to-1 chamfer, so if you repair a 3-inch diameter hole through a 1/2-inch-thick hull from the outside, you end up with about 15 inches (diameter) of surface damage to refinish. Repair it from the inside and you have only a 3-inch hole to refinish.<<< 15 inch hole , quoted from the casey page,,, and nowhere did i say that poly adheres stronger then epoxy i said the mechanical bond is bout same hehe ( how deep into the poly does epoxy melt into, and if it does that isnt it a chemical bond?),, just because something is stronger or does something better doesnt mean its ALWAYS the right answer,,and if your any kind of fiberglasser you know there is shrinkage and you deal with that and it ends up right,,,,,,take a 3 foot square of glass,,lay it up with poly,,, cut a hole bout say,,5 inches,,repair it with epoxy,, now take it and bend it,,,,,what flies?,, that epoxy patch does,,,,now do the same with epoxy and glass,,,fix the hole with epoxy,, bend,,, what flies??? nothing.
|11-11-2007 07:30 PM|
Originally Posted by the1much View Post
For normal repairs, I have seen the high shrinkage of a polyester repair cause many more "cracking" probelms than any flexing issues with epoxy. I agree that epoxy is harder, but I would also suspect that you would have more issues with stiffness or flexing due to the fact that the typical repair is made of a differant layup material than the original hull, ie a patch made with biaxial glass will be more rigid than a chopper gun hull, regardless of the resin used.
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