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  #1  
Old 08-23-2010
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Bondo Glass???

I've ran into this product called bondo glass- smells like bondo- looks like its loaded with a billion fiberglass strands and says its water proof.

has anyone tried using this for fiberglass nicks and dings? Im thinking using that as a filler (not in any type of structual way) then laying some epoxy as double insurance that it will keep water out.
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Old 08-23-2010
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Well, I can't pull up decent tech specs from here, but if it's polyester based (as is normal Bondo) then you might want to use a polyester resin rather than epoxy resin, as epoxy doesn't generally like polyester (they don't form a particularly impressive bond, unless you jump through some silly hoops first)

On the other hand, using any polyester based resin can lead to osmosis issues if it's left exposed underwater for a considerable time.

I usually like to just repair boats with what they were made with.. which is usually polyester
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Old 08-23-2010
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so if its under water, ill obviously be painting it which pretty well keeps it from the water (unless it cracks) i should be ok...?
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Old 08-23-2010
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so digging up some more info on resigns i learned this:\

Epoxy resin is known in the marine industry for its incredible toughness and bonding strength. Quality epoxy resins stick to other materials with 2,000-p.s.i. vs. only 500-p.s.i. for vinylester resins and even less for polyesters. In areas that must be able to flex and strain WITH the fibers without micro-fracturing, epoxy resins offer much greater capability. Cured epoxy tends to be very resistant to moisture absorption. Epoxy resin will bond dissimilar or already-cured materials which makes repair work that is very reliable and strong. Epoxy actually bonds to all sorts of fibers very well and also offers excellent results in repair-ability when it is used to bond two different materials together. Initally, epoxy resin is much more difficult to work with and requires additional skill by the technicians who handle it.

Vinylester resins are stronger than polyester resins and cheaper than epoxy resins. Vinylester resins utilize a polyester resin type of cross-linking molecules in the bonding process. Vinylester is a hybrid form of polyester resin which has been toughened with epoxy molecules within the main moleculer structure. Vinyester resins offer better resistance to moisture absorption than polyester resins but it's downside is in the use of liquid styrene to thin it out (not good to breath that stuff) and its sensitivity to atmospheric moisture and temperature. Sometimes it won't cure if the atmospheric conditions are not right. It also has difficulty in bonding dissimilar and already-cured materials. It is not unusual for repair patches on vinylester resin canoes to delaminate or peel off. As vinylester resin ages, it becomes a different resin (due to it's continual curing as it ages) so new vinylester resin sometimes resists bonding to your older canoe, or will bond and then later peel off at a bad time. It is also known that vinylester resins bond very well to fiberglass, but offer a poor bond to kevlar and carbon fibers due to the nature of those two more exotic fibers. Due to the touchy nature of vinylester resin, careful surface preparation is necessary if reasonable adhesion is desired for any repair work.

Polyester resin is the cheapest resin available in the marine industry and offers the poorest adhesion, has the highest water absorption, highest shrinkage, and high VOC's. Polyester resin is only compatible with fiberglass fibers and is best suited to building things that are not weight sensitive. It is also not tough and fractures easily. Polyesters tend to end up with micro-cracks and are tough to re-bond and suffer from osmotic blistering when untreated by an epoxy resin barrier to water. This is really cheap stuff.
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Epoxy sticks to polyester just fine - better than polyester sticks to polyester as a matter of fact. Polyester bonds to polyester well when the original structure is still green, either in the mold or having just been pulled from the mold. But for a finished boat like any of ours epoxy has much better adhesion. That is why all good repairs to a fiberglass (polyester) boat are done with epoxy. I can't think of any reason to use polyester based
products to repair a fiberglass boat except for possibly price - and since epoxy sticks better it will probably last longer as well, possibly making it a better value as well. Now gelcoat is reportedly not good at sticking to epoxy -although the Gougeon Brothers disagree and have had good success with gelcoat over epoxy. Epoxy is easier to work with as well. And you're correct - epoxy is much better at keeping water out of the laminate causing blisters and osmosis. Barrier coatings like Interprotect 2000 are epoxy products and they stick very well to polyester.
I wouldn't use anything like bondo on a boat - it's good for a cheap fix on a car though.
I have been using epoxy, both West and other brands, successfully for over 20 years - I have never had a problem with adhesion or anything else for that matter. It can be thickened to any consistency required by adding different materials from colloidal silica to wood flour to microballoons to gypsum. Here's a link to a good manual on epoxy and its uses. From West but it applies to all brands except for mix ratio in some cases. The product manual is at the top of the page and specific repairs are below - all can be downloaded.
Fiberglass Boat Repair and Restoration
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Last edited by mitiempo; 08-23-2010 at 10:56 PM. Reason: add
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I don't disagree with anything posted above. Except the claim that epoxy adheres to everything... clearly they've never tried to gel coat an epoxy-based repair job.

The hardness of epoxy is not a feature, it's just a characteristic. If your boat is laid up with polyester (which most are) and one section of it is repaired with epoxy, then what you have is a hull with a certain amount of flex, and a small spot with a lot less flex. It doesn't take much consideration to realize that the bond between the two is going to be compromised early and often. Also, a hard thing being squeezed around inside a not-as-hard thing is likely to pop out.. not a handy feature for the average repair.

Here's the thing.. the article quoted makes it sound like the only factor that matters is finish hardness/bond strength. But if that were the case, why wouldn't we all just build everything out of carbon fiber? After all, the weakest carbon fiber build is much stronger than the best fiberglass build.

Vinylester is excellent stuff. That's why it costs twice what polyester does. And if you are going to put a proper barrier coat between the boat and the water, then the tendency for polyester to wick water won't come into play.

Plus, if you are repairing anything more than about 5% of your boat, the savings in using polyester rather than vinylester will pay for the entire barrier coat.

Also, of course you have to wear a respirator. VOC's are present in all those chemicals. Knocking polyester because of VOC's is like knocking sports-cars because they have seatbelts. All cars require seatbelts.. and all resins require respirators.

I note that the article didn't even mention acquired epoxy allergies...

Anyway, I'm just one dude on the internet. I don't even believe myself, half the time. Call up your boat builder, and ask them what they used to lay up your hull. Ask them why. They know a ton more about boat building/fiberglassing than does some dude on the internet.

If you want me to, though, I can go on for hours about this stuff.. but I've used enough white space already
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See, there you go. mitiempo has been doing this for a long time, and he's had great luck with epoxy. I've spent some time with fiberglass, and had great luck with both vinylester and polyester.

The main reason I would suggest polyester for boat repairs is that the boat was originally built with polyester. Obviously, if you have a rare exception to that rule, then go with whatever it was built with. Chances are, it wasn't epoxy.

It's always good to have options
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no, I seriously appreciate perspective, I dug a little deeper and it said this stuff is a "unsaturated polyester" im not sure if thats good or bad, i cant seem to find anything on it.

here is the deal, 1977 catalina- has the full keel, its cast iron, or maybe it lead..anyway the coat on top has cracked in a few spots, ive opened it up so it doesnt have the hollow knocking sound, and just want to patch the hole to a water tight quality. looks really mean nothing- its under the water (unless on the trailer) so i need to fill the 1/4" void and make sure its not going to rust my keel anymore

also im not referring to the "Catalina smile" that's another issue on its own
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Old 08-23-2010
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see, this is where a few pictures would be awesome
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Old 08-23-2010
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agreed- awesome they would be, but my camera just walked home with my girlfriend, the best i can explain this is that there is a gelcoat on top of this keel and it flaking up in a few spots, kinda like a paint chip (but not as severe) small 3 inch cracks appear and obviously the keel gets wet and rusts, so im trying to open and seal up those cracks to prevent any further damage.
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