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I have a client that has a hairline crack in his bass bridge, which I pointed out to him. It runs through approximately 6 bridge pins (on the speaking length side/front/top side of the bass bridge).

To cut a long story short, he would like the bridge repaired with epoxy.

Advise and opinion on a few things will be greatly appreciated,

1. Which is better to use, epoxy or CA glue?
2. If one did use epoxy, how does one get the epoxy into the hairline crack (and a sufficient amount of it)?

For point 2, I recommended to my client that I would use a putty knife (similar in one sense to an artists pallette knife) and push the epoxy into the crack as I have read similar talk on the forum. My client disagrees with me saying the viscosity is to thick and has requested that I look further into the matter of how to get the epoxy into the hairline crack as this is the route he would like to go.

Thank you,

Regards,

Slacken the bass strings on the affected bridge pins. Remove the strings from the hitch pins and set them off to one side. Remove the bridge pins in the affected area, plus a couple more beyond. (The damage may extend beyond what you are able to see, so you will treat slightly beyond.)

Mix a small amount of epoxy, I prefer a slow setting like "System 3" 2-1 slow ratio. Using something smaller than the removed bridge pins, like a piece of music wire, swab the holes thoroughly with epoxy. try replacing a couple of the bridge pins. If you have enough epoxy in the holes the hydrostatic pressure from re-inserting the pins will force the epoxy into the cracks and up to the surface. Clean up the excess. Let it cure for 24 hours. Come back and replace the bass strings on their hitch pins and tune several times.

Thank you very much Dale! I am going to recommend this to my client. I think that he will see what you are saying and agree with your procedure.

Regards,

Mark,

If you do the repair, I'd be very interested in which brand and type of epoxy you will use. I've thought about this, and seeing that I have a similar repair to do on one of my pianos, I've shopped around for epoxies. It appears that the brands which are typically discussed on these forums (e.g. "West System" or "System 3"), are not available here in South Africa. I've seen Pratley "Quickset", which comes as a white paste in two tubes, or as two packets of "clay". And then I've seen various clear "5 minute" epoxies, e.g. by Bostik, that come in two parallel syringes. But I think, a slower curing epoxy would be better... (as Dale already said.)

Which of our local options is best to use, I really couldn't say, hence my question to you. If you find something that works, please let me know!

Thanks, and regards,
Mark

Mark,

Actually, you were the person I was going to contact about what epoxy to use.

Yes, I will use a slow drying epoxy.

I have seen various slow drying epoxy products around, but don't remember the product name offhand.

When I do find something that I think will do the job, I will let you know.

Regards,

I have seen enough failed cracked bridge repairs to come to the conclusion that the real, long term solution to this is bridge re-capping. Of course it all depends on the situation: the type of piano, age, value, musical expectations, budget etc etc. If the piano has any lasting value, I would tend to lean toward re-capping, which is not that difficult in the bass.

I've had bad luck with attempts at epoxy repairs. Over the course of time there just doesn't seem to be any product that stands up to the kind of stress that a bass bridge is subject to.

I have a friend who has completed hundreds of successful epoxy bridge repairs over the years. If done properly it will not fail and will outlive the piano. The appropriate high-strength marine epoxies are extremely hard and will soak into the wood making highly durable repair. Failures probably come from using stale epoxy, using epoxy that is too thick and thus it doesn't penetrate into the wood, and not using enough. I like to let the repair sit for at least 24 hours before putting stress on it.

Upright pianos need to be on their back, preferably with bottom of piano removed.

My friend taught me to use a veternarian syringe to inject the epoxy and fill the bridge pin holes completely. After a couple of minutes you may see the epoxy disappear as it soaks into the wood and cracks below the surface. Add more if necessary. When the pins are reinserted the epoxy will hydraulic into any thin cracks. It's often necessary to apply some clamps to the bridge to pull it back solidly together.

It is very important to mask things off well - this is a very messy job, and epoxy is nasty stuff to work with. Cheese cloth works well for cleaning all the squeeze out - use plenty. You can "floss" the bridge pins, and the fibers of the cheese cloth are good at picking up the epoxy.

This is why it is very important to use the slow hardener - especially if you don't do it very often. You should give yourself a good hour of working time. It also allows more time for the epoxy to soak in and meld with the wood.

I agree. The quality of the epoxy is crucial. Preparation and cleanliness also. The longer an epoxy takes to cure the stronger it is. And it is.

For hairline cracks, without taking out the pin, as a precautionnary measure, CA works great, put prep and cleanliness is just as crucial.

All the best.

Find a local marine supplier. They will have appropriate epoxies. Slower is better. A little time will allow the epoxy to fully bond with the substrate. The thicker, pastey versions will not work for this repair. And BTW, it's "curing" not drying. http://www.systemthree.com/sys3news/?p=79 System three manual at this URL


[color:#333333]The following is excerpted from The Epoxy Book, found at the URL above. It is in the section VII D bonding with epoxy.. Pay special attention to sections 3 and 4 which have a more direct application to the repair you are looking at.


First, remember that the ultimate strength of any glue joint is a function of the glue surface area. The more surface area, the stronger the joint. This is the reason that scarf joints are made at a minimum 8:1 slope. Fillets increase glue surface area and are used to relieve stress concentrations that build at right angle corners. Stringers, for example, should have fillets where they butt onto the boat hull planking.

Second, make sure that the surfaces being glued are clean, free of grease, oil, wax, and other contaminants that could act as release agents. If the surface is coated with cured epoxy, sand before gluing and wipe the dust off. Prior to sanding wipe away any oil or grease with a clean rag and suitable solvent. Remove paint rather than trying to glue onto a painted surface. Epoxy resins stick well to sanded paint but the overall bond strength will be no better than the paint to substrate bond.

Third, do not over-clamp. Epoxy resins require only contact pressure. Over-clamping can squeeze most of the adhesive out of the glue joint and the epoxy that is left is absorbed into the wood starving the joint. A glue-starved joint is very weak. Use only enough pressure to hold the joint immobile and keep the two surfaces in contact until the epoxy has set overnight at normal temperatures. Nails, screws, clamps, rubber bands, or staples can all be utilized. Clamp just hard enough to close up the joint.

Fourth, remember that epoxy resins continue to cure and build strength for several days after they solidify. Joints that will be under immediate stress once they are unclamped need more cure time before the clamps are removed. Overnight cures are usually sufficient for most non-stressed joints. In cold weather the time could stretch out to several days. A common cause of epoxy joint failures is excessive stress before the epoxy has reached sufficient strength. This might occur when a scarf joint is bent too soon.

Fifth, protect the finished glue joint from weather degradation. Wood that is allowed to weather will cycle through moisture content extremes. Wood expands as the moisture content increases. This expansion can set up enormous stress concentrations across a glue joint due to uneven rates of expansion on either side of the glue line. These stress concentrations can exceed the strength of any glue, including epoxy resins, causing failure. Protect the joint by epoxy coating all surfaces of the glued wood. This will stop moisture cycling and prevent failure because of weathering. This is not a problem for wood glued with epoxy that will not be subject to deep moisture cycling [/color]

I don't like what the epoxy does to the tone, unless it was just an impression, some sort of damping

Thanks to all for your input.

Dale, my client has accepted the procedure you recommended, now I just need to find the correct epoxy. And thanks for the further informative post too.

Ryan, thank you for your helpful post too.

Regards,

I have found a supplier of West System in South Africa (albeit an industrial supplier - not sure whether they supply to private individuals).

To anyone here who has used the West System: Mark and I would be very interested in your specific "recipe": starting with resin 105, and then...
... which and how much of the hardeners (presumably 209 with 1 hr. pot life?),
... which and how much additive (any?) and/or
... which and how much of the fillers (402-406, presumably 403 for general wood or 404 for high load?)

See also:
West System product selection chart at Wessex Resins, UK
West System product selection chart at West System, USA

Any information appreciated.

Mark,

Look at Sika and epidermix(abe) epoxy products.

They seem to be make quite a range.

Regards,

Mark,

Both the Sikadur and the epidermix ranges seem to be formulated primarily for construction (concrete, bitumen, glass, rubber, etc.)

While I would expect them to stick to wood just as well, I really can't say for sure.

From the epidermix range, only the 316 makes sense to me, because is pourable, i.e. thin enough. The others are gel-like pastes.

Mark,

The Sika product though it seems to lean more towards, as you say, formulated primarily for construction (concrete, bitumen, glass, rubber, etc.), says the following in it's data sheet,

"High-modulus, low-viscosity, high-strength, extended pot life, epoxy adhesive. Sikadur 35, Hi-Mod LV LPL, is a 2-component, 100% solids, moisture-tolerant, low-viscosity, high-strength, multi-purpose, epoxy resin adhesive. Low pressure and high pressure injection of cracks in structural concrete, masonry, wood, etc."

So, I am hoping that this will be strong enough for bridge repair.

Thank you.

What is said by historical furniture restorers is that epoxy does not follow the wood with moisture changes, so can be the source of weakness at the frontier between impregnation and non impregnation. Hence the use of vacuum pumps to strengthen old wood, that is then no longer wood but a composite.

Indeed such trouble if prone to happen may take years or decade, but that may limit the use in certain cases.
Personally I trust the strength of the product, but not its hardness.
Even well cured, the epoxy remain somehow supple (unles "cooked" so the process is complete between the 2 molecules ) my impression is that the epoxy coating is adsorbing a part of the high partials energy. I would use it to repair a bridge eventually, on a lesser grade instrument.

I heard of epoxy impregnation for bridge top, but as the tone of those instruments does not suit me, i cannot say how good or no it is for tone.

Other resins may also apply I suppose if it is for non structural work just for hardening the top.

I was told that heat can be used to harden wood.

Heard of any process like that for bridge caps ?


to work epoxy thru the wood is not hard, most epoxies will flow a lot the problem is more to avoid it to go out from other orifices. if too thick the heat can be used to thin it.

Old resin have to be " cooked" before use it allows the cristals that depose at the bottom of the can to mix again. I was given 60 c heat for a little time.

I heard of the use of 10 years old resin for boat making, by a technician of my epoxy provider without trouble.

I use the 209 extra slow hardener with the West System epoxy. I also use the pumps so that you get the right ratio of epoxy to hardener.

Don't add anything to it, because any fillers will have a negative effect on its penetrating properties. I use a hypodermic to inject the epoxy into the holes. It needs to be a pretty big needle - like what you would find at a feed store for injecting cattle.

Of course you should practice with it before using it on a piano, so you can get a feel for how it works.

Kamin says he questions the hardness - that depends entirely on the epoxy you are using. The stuff I'm using is very hard - harder than the wood.

Thank you Ryan for your help and input.

Regards,