# Bondo Glass???



## 4cylindersoffury (Aug 21, 2010)

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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## CapTim (Aug 18, 2009)

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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## 4cylindersoffury (Aug 21, 2010)

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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## 4cylindersoffury (Aug 21, 2010)

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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## mitiempo (Sep 19, 2008)

CapTim
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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## CapTim (Aug 18, 2009)

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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## CapTim (Aug 18, 2009)

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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## 4cylindersoffury (Aug 21, 2010)

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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## CapTim (Aug 18, 2009)

see, this is where a few pictures would be awesome


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## 4cylindersoffury (Aug 21, 2010)

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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## Maine Sail (Jan 6, 2003)

CapTim said:


> 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.
> *


Actually, I've done it many times. West Systems has addressed this in a white paper article. The myth that polyester can't be used over epoxy is basically just that. Considering I have done this many times, and never once had an ounce of issues, I know it to not be true. Gelcoat sticks just fine to *properly prepped* epoxy. If you feel like using polyester that will work too, and will cure faster.

If you read the article you will see that the gelcoat is still bonding at well over 500 PSI or better even when submitted to 100% humidity in a 100F room, for many weeks. In most cases it is bonding at over 1000 PSI.

*West Systems Polyester Over Epoxy*
_*
"Our test clearly demonstrates that polyester
gelcoat will bond to a properly prepared
WEST SYSTEM® epoxy as well as to a cured
polyester laminate. This test confirms that
polyester gelcoats can be applied over cured
WEST SYSTEM epoxy on repairs below the
waterline."

*_I use vinylester, polyester and epoxy. If I want and need strength in a secondary bond situation I use epoxy.


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## mitiempo (Sep 19, 2008)

CapTim
Epoxy is hard but it is much more flexible than polyester. And it has better adhesion to wood or polyester than polyester does even to itself. That is one reason that a cold molded wooden boat is sheathed with epoxy resins and cloth, not polyester. Epoxy can stretch more than polyester without losing tits bond to the substrate. The only thing I disagree with above is the statement that "epoxy resin is much more difficult to work with" - I find it very easy to work with. From the tailoring of pot life by using different hardeners to its ability to be mixed with many substances for differing consistencies. And the results are excellent.

Gelcoating epoxy has nothing to do with epoxy sticking to anything but with gelcoat's ability to stick to epoxy.

By the way, it's not luck I have, it's a little bit of skill and a great product - any brand of epoxy I have tried.


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## CalebD (Jan 11, 2008)

Epoxy has better adhesion qualities as well as water resistance qualities then vinyl, period. Yes, it is harder and stiffer and a bit more permanent then vinyl based resins but that is why it is used for a lot of repairs. I've even applied vinyl gel coat over a West System epoxy repair to our hull deck joint that has been good for nearly 10 years. Yes, every can of gel coat I've ever looked at and read the label of said not to apply it over an epoxy repair. 
Yes, epoxy is harder and stronger then vinyl resin and doing repairs with epoxy might cause or create a hard spot in a hull that should flex more then epoxy does. 
I think that the idea that epoxy repairs to a hull will not flex is debunked by the idea that this is how most catastrophic hull repairs are done using epoxy rather then Polyester resin. If you don't want your repair patch to adhere very strongly on your hull then go ahead and use a Polyester resin. See how that works out for you and please report back to us after a Beaufort Force 8 or above storm. I am dieing to know if it works or not. 
Epoxy has about 4x the adhesion qualities of vinyl resins and the reason it is used is because it gives peace of mind that it can withstand a blow or two and not give up the ship. 
On the hull of my boat I use epoxy to repair divots and cracks that I don't want falling out due to hull flexing. The whole point of the hull is to keep the water on the outside and not let it inside.
Sorry Capt. Tim, you make some good points but I am not convinced one bit even though I know the builders of my 43 year old boat used a crude vinyl resin to make her hull. It is still very thick and sound and the small amount of relatively hard epoxy repairs amounts to little more then a big nothing in terms of my vessels sea worthiness.


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## 4cylindersoffury (Aug 21, 2010)

so maybe like epoxy 105 with a filler additive? something that will give it some mass to fill?


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## mitiempo (Sep 19, 2008)

Agreed, and lest we forget the original poster and his question, underwater I would use epoxy as nothing can compare when immersed constantly. I would use West 105 with 206 hardener which will give you a bit more time to work with it before it sets. I would mix it with colloidal silica. The link I posted gives you all the information you need in their manual. And if you have a question they even answer the phone at Gougeon Brothers.


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## seabreeze_97 (Apr 30, 2006)

"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."

Wouldn't that be gel coat not adhering to epoxy?


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## AllThumbs (Jul 12, 2008)

I have used bondo to fix dings in a fiberglass boat and it works just fine. No colour matching is possible tho, so it will need to be painted.

Eric


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## SVImpunity (Mar 7, 2009)

*Bondo*

There is a product that I have used & is sold at West Marine that is structural fibreglass filler. It has many chopped FG strands & is meant for filling voids. It is Polyester Resin based & provided the repair area is well prepared & cleaned it will do the job well. It doesn't slump as it's a fairly stiff consistency. The other possibility is to mix a batch of epoxy resin & thicken it with filler balls or talc.


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## mitiempo (Sep 19, 2008)

Underwater the proper additive to epoxy is silica as the microspheres and many other thickeners are porous to water.


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## sailingdog (Mar 19, 2006)

*I'd point out that for the average boat owner, who is not a skilled boat repair person, using epoxy resin is far simpler and more forgiving than using polyester resin. * First, epoxy resins have a far greater secondary bonding (adhesive) strength than do polyester resins-making for a stronger repair. Second, the mixing of epoxy resins, usually a simple 1:1, 2:1 or 4:1 ratio, is far simpler and more forgiving than mixing polyester resin and catalyst. Third, epoxy resin is far more impermeable to water, and as such will protect a repaired area far better from moisture intrusion.

The fact that epoxy resin-based GRP has slightly different characteristics than polyester resin-based GRP in terms of tensile strength, flexibility and such means little if you make the repair with the recommended 12:1 taper, which allows the materials to transition gradually, rather than have a sharp difference with a hard edge.


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## mitiempo (Sep 19, 2008)

And less smell.


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## SlowButSteady (Feb 17, 2010)

I've used Bondo-glass to repair GRP (fiberglass/polyester) several times. There are two types, long-stranded (which is actually called Bondo-hair) and short-stranded. It bonds just fine to sanded and prepped GRP. My only concern is that when sanded the surface will have glass fibers sticking out, albeit on a microscopic scale. To prevent these fibers from wicking water into the Bondo, and maybe through to the bond with the the underlaying surface in really thin parts of the patch, after the patch is hard and sanded smooth I make sure to add a layer of plain resin (catalyzed, of course) or epoxy to the outside of the patch. This may be overkill (is there a hyphen in anal-retentive?), but I treat it like it was roving and make sure it is completely sealed.


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## mitiempo (Sep 19, 2008)

You're right to seal it. Better with epoxy to keep the water out. The same should be done with thickened epoxy used underwater to seal it. Especially if a porous filler like microballoons is used.


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## mitiempo (Sep 19, 2008)

CapTim
Here's a link you might find interesting - and others might as well. It is a simple but thorough description of the properties of epoxy and polyester when repairing polyester (fiberglass) boats.
http://www.westsystem.com/ss/assets/Uploads/Ew22Fiberglass.pdf


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## Boatsmith (May 3, 2009)

The general rule is that you can put epoxy over poly but not poly over epoxy. That being said we do gelcoat over epoxy repairs. Care must be taken to remove any trace of amine or you will have problems. With proper prep we've not had any gelcoat over epoxy failures. Epoxy is certainly our choice when making any type of secondary bonds. All repairs are secondary bonds. You can mixed chopped fiber into epoxy to make your own epoxy "kitty hair". A coat or two of unthickened epoxy over the repair are will prevent any wicking


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