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post #1 of 9 Old 05-24-2012 Thread Starter
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Bronze and West system epoxy

Does west system epoxy with collodial silica (406) adhere to bronze for a below waterline application? I have read at west system that it works below waterline just not sure on the adhere to bronze. Any thoughts would be helpful.
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post #2 of 9 Old 05-24-2012
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Re: Bronze and West system epoxy

From my experience epoxy to metal is not going to work long term structurally. It will work ,well with metal embedded. Kind of like when a prop strut is replaced. I could be wrong but I think water would get between epoxy and metal over the long term. the way to prevent that, would be to seal the the edge with a poly sulfide caulk.

here's a link to a page on West Systems. all metals were tested except bronze although it seems all metals are roughed up with 80 grit for them to bond.
Also, they make a bonding system.
#860 Which was used in some of the tests.
WEST SYSTEM - Projects - Effects of surface treatments on adhesion to metals

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post #3 of 9 Old 05-24-2012
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Re: Bronze and West system epoxy

Looking at those test results. I'd say the adhesion is inversely proportional to the metal hardness. Qualitatively, I'd say the sanding with 80 grit makes deeper scratches in soft metal. So, the adhesion seems related to the "tooth" generated by the scratches so you might want to make deep scratches in your bronze. Use a file to scratch it after you have done the 80 grit.
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Re: Bronze and West system epoxy

I'd say no. It won't.

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post #5 of 9 Old 05-25-2012
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Re: Bronze and West system epoxy

West has free tech siupport on the phone. I'd expect they are closed until next Tuesday, but they'll gladly give you the details and you can rely on what they say.
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post #6 of 9 Old 05-25-2012
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Re: Bronze and West system epoxy

There are two different situation that could be going on here, and I am not sure which to answer. But they actually both result in the same answer, so the short answer is don't do it. Epoxy bedding metals is almost never a good answer for a umber of reasons.

First, and assuming we are talking about a through hull fitting, it is never proper to epoxy bond a through hull fitting into the hull. When used in this application the epoxy becomes a structual element of the hull itself, and in most cases is actually stronger than the surrounding hull structure. This means there is no way to remove the through hull in the future except cut an even larger hole in the boat. Typically these future holes will be so large that they can't simply be filled, but instead require a full fiberglass filled patch job, which can be expensive, time consuming, and unnecessary.

Instead the proper way to bond a though hull into a boat is by first drilling an apropriate sized hole in the hull, then epoxy sealing the edges of that hole with neat (unthickened) epoxy. Finally you insert the flange into the hole, after applying siKaflex or some other Semi-structual caulk (4200, ect) to the flange of the through hull. This sealant is what makes the system water tight, not epoxy. Since it can be removed (though with great difficulty) in the future if it becomes necessary. Remember most of the water will be excluded by the force applied y the backing plate/screw fitting, the sealant just needs to prevent the drips.

Basically here the sikaflex is used as a gasket, while the load is taken by the bolt. Same reason you don't need to epoxy seal oil filters, or impeller housings.

A longer and more detailed set of directions can be found at Replacing Thru-Hulls and Seacocks Photo Gallery by Compass Marine at . This also avoids the problems discussed below.

If this is not for a through hull, but just attempting to bond bronze to fiberglass again epoxy is not the proper tool. This has to do with the characteristics of epoxy and pretty much all metals (though some are better than others). The issue is that while epoxy is an incredibly strong material, and will adhere to almost everything, is it also very brittle, so it can't flex or expand much. Of course nether can bronze, so we are trying to bond two very hard inflexible objects... This would be fine if they were dimensionally stable, but they aren't.

As the materials heat up and cool they actually change size. And since there is no flex in either of these materials the bond where they are joined will be put under tremendous preassure, since the force that is being opposed is found at the atomic level. So how bad is it? Well the change in size combined for the materials is .000,041 inch/inch/degree F. Not a lot really, but more than enough with cycle loading to rupture the joint between the two materials.

This is most easily observed when you apply a torch to stuck nuts To break them free. The heat causes the nut to expand faster than the bolt does, allowing you to break it free.

This requires us to instead join bronze with something more flexible like sikaflex, or 4200, even west systems G-flex. Basically something that has a little bit of give to reduce this thermal expansion load.
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post #7 of 9 Old 06-17-2019
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Re: Bronze and West system epoxy

Looking for advice in a somewhat related topic. My boat has a shaft log wall with what I believe is ~5mm (.20"). There is (was) a small nipple threaded in with a small amount of structural adhesive for flushing water for the shaft seal. The nipple was substandard bronze or brass which crumbled in my hand and started leaking. My thoughts on a repair would be to glass in a fiberglass block on the shaft log to build up some thickness. Then, I was originally thinking of threading a hole and use a bronze hose barb into the block. Using threads in fiberglass doesn't provide much strength. Anyways, there are no commercial fittings down this small anyways. My other idea is to bond in a bronze tube into the block machined with a barb on one end. Maybe this is reasonable except do not use epoxy to bond the tube. I'm concerned with expansion and contraction of the dissimilar metals, a crack would develop and enough corrosion would start to develop a leak. Maybe use the tube with 3M 5200 to ensure the bond is never broken to initiate any corrosion. I have to pull the boat out of the water early to do this repair, so want to get it right the first time. Any advice is appreciated.

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post #8 of 9 Old 06-17-2019
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Re: Bronze and West system epoxy

why do you need a flush line ? what size shaft and type of seal is it. most small sailboat shaft logs do not need a flush, not going fast enough to create a suction in the tube. dripless seals have a fitting to remove the air to keep the seal wet.

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post #9 of 9 Old 06-18-2019
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Re: Bronze and West system epoxy

You sure it's not a vent line to relieve pressure, so that water can enter the log and lubricate the shaft seal? Anyway, I'm not sure you need to build it up thicker. A good bronze nipple and 5200 sounded like a decent idea to me.

On the other hand, are you sure you need a vent at all? Often they are for powerboats, who go faster and create a vacuum at the back of the log, which limits water entry. I think the recommendation for vents is at around 10 knots.

Just a random thought, be sure to check yours out, if this is what you have.

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