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  #1  
Old 10-06-2010
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poly or epoxy

I'm repairing my rudder that has several cracks in it. After i've filled all the cracks I would like to wrap it with fiberglass. Now the question is does it matter if I use poly resin or epoxy resin? What kind of fiberglass should I use? Mat or weave? What weight? Any help would be great. After I finish with the fiberglass should I use a epoxy barrier coat. If so how many coats do I need. This is a fresh water boat.

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Old 10-07-2010
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In general, structural repairs should be done with epoxy, especially if you're not proficient or familiar with polyester and vinylester resins and their use. This is for several reasons.

First, epoxy resins are generally easier to mix and more forgiving of small errors in measurement than polyester/vinylester resins. Most epoxy resins are a 1:1, 1:2, 1:3, or 1:4 ratio and are not dependent on temperature or other variables for changing the mixture ratio, which can be the case with polyester/vinylester resins and their catalysts.

Second, epoxy resins have far greater secondary (adhesive) bonding characteristics than do polyester or vinylester resins. This means that the bond between the repair and the original material will generally be stronger when using an epoxy resin.

Third, epoxy resin is more resistant to water or moisture intrusion, since it is generally less permeable to water than vinylester or polyester resins are. Polyester resins are generally the worst with regards to osmosis or water permeation. Vinylester resins are better than polyester resins.

I would use 6 oz. or 10 oz. cloth, since you're really not trying to build a lot of thickness or strength here. Ideally, you should use a layer or two of kevlar under a couple layers fiberglass, then fair the fiberglass with thickened epoxy. The reason for this is simple—kevlar cloth has very good abrasion resistance and it will make the rudder far more damage resistant than just fiberglass cloth alone. However, kevlar cloth has a nasty tendency to float in most resins and is impossible to sand fair, so putting a couple layers of fiberglass over it is usually necessary.

You shouldn't need a barrier coat, since the epoxy layup is pretty good at resisting osmosis.
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Old 10-07-2010
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Use epoxy because it has much better adhesion than polyester. The cracks can be filled with thickened epoxy. Mat by itself has no great strength and most mat is not epoxy compatible. If additional strength is desired use biaxial which is stitched roving with epoxy compatible mat on one side. It is available in different weights. If additional strength is not needed a light to medium weight cloth is all that is necessary. Epoxy is its own barrier coat and after glassing a few coats of straight epoxy is all that is required before bottom painting. Here's a link to West's fiberglass repair manual which is downloadable. Excepting for mixing proportions of the brand you are using it will apply to any brand of epoxy. Fiberglass Boat Repair and Restoration
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Old 10-07-2010
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I'd quote both previous fellows.

Whenever your wallet allows for it, go for epoxy...
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