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Filling inaccessible cracks with a liquid filler?

1315 Views 24 Replies 11 Participants Last post by  davezedlee
I am working on a musical instrument that I designed. It has a very complex internal structure, and I fabricated it by cutting cross sections on a laser cutter, and then gluing the pieces together with wood glue. It is currently in two halves, and I am ready to join them together. However, before doing so, I would like to ensure that any potential cracks between layers are completely sealed so that air cannot bypass the intended path. The surface to seal is charred by the laser cutter, and inaccessible due to the narrow deep gaps in the design.

My thought is to flood the cavity with some sort of liquid such as boiled linseed oil + turpentine, or wood glue thinned by water and then draining out the liquid and spinning the work as it cures. Some sort of thin epoxy with a very long working time would be nice, but don't know if such a product exists.

I tried googling for information to help me proceed, but this is a rather odd case that didn't turn up relevant solutions.

Any advice from the experts here on how I should proceed?
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I'm not sure what your answer is but I do know boiled linseed oil is not a good one. That stuff takes a long time to dry. Months or years adding turpentine will make your drying time longer.
How big of a cavity you working with will definitely steer your choices.

Good Luck
Fish glue might be an option. Very long open time. It would be easier if we knew what you were building along with some pictures.
We don't know if this is a wind, string, or percussion instrument. It might make a difference.
+1 BLO is bad idea.
Using any hardening oil as sealer will take forever to cure.

Sorry not enough information to suggest actual materials?
What kind of instrument; wind, string, percussion?
Do you care about tone added by sealer?
How well do you want to seal any gaps?
Does it have to be 100% liquid tight with 100psi applied or just room temp air pressure sealed?
Soo many questions unanswered…

There are all kinds of gap and/or pressure vessel sealers available.
The soft ones will dampen vibrations, hard coatings will resonant differently than wood.
For a pressure tight seal could immerse the pieces in pressure pot of resin (cactus juice?), drain/remove excess, and then bake the structure into one solid mass?
etc, etc, etc….
Massive number of options, IMHO,
But then I AM a materials engineer who has worked with adhesives/coatings/polymers all my life. LOL

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It is a wood wind instrument. A hard finish is definitely preferred to soft for tonal quality. The wood is Baltic Birtch plywood.

Potential cracks would be thin, but definitely need to be airtight under minimal amounts of back pressure that can be created with the lungs pushing air through a very narrow channel with a cross section of about 1/32" by 3/8". That narrow cross section is external, and in no danger of getting filled by the sealer. The narrowest intended gap on the inside of the cavity is about 3/16" wide.

I will take a picture of the central cross section and upload it.
Circle Font Pattern Symmetry Art

This is what the central cross section looks like. It is 4" deep. The outside diameter is about 9.5".


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I went ahead and unclamped the work so you can get a better idea of the internal surface finish and overall scope.

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Wow, PIA to seal with real sealer, and not change the tone?
See why you thought about BLO 1st. :)

Is there any way to test the chamber sealing?
How about this:
Place a 1/4 thick piece of EDM rubber on top, coated with temporary spray adhesive, with small holes drilled in each chamber location. Use a dispensing syringe with blunt needle, remove the internal stopper, so compressed air flows into chamber, and listen for leaks between chambers.

If there are no leaks, then don't need to seal it?

Have used dispensing needle with foot switch apparatus in development lab for verifying adhesive sealing on automotive pressure transducers. :)

Regardless, for a 2 atm human pressure rated seal, my brain thinks might need an immersion process if you have numerous leak issues?

Simple version if all cracks are exposed, and not large gaps:
Dip in it 2lb cut of shellac, wait for bubbles to stop rising up, drain/shake remove bulk liquid, wipe/brush out any remaining, dry a couple days, and sand the glue joint to ensure glue can stick to wood?

If you have larger gaps, then check out cactus juice wood stabilizer?

Just tossing ideas that I might use in your situation. Only instrument I've made was a ukulele. Although, I did help make a part that looked like that? Was micro machined inertia sensors, made via etching grooves deep into silicon die about 1/8" square using photo lithography? While lot smaller! LOL

Best Luck!
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I can only speak from speaker building on air-tight. Yo want it sealed well enough the mass of the air in a crack does not matter, but you do not want it sealed as air pressure differences. A pinhole is good. Nail hole is not. A leak with a longer time constant than the lowest note wil be irrelevant.
Curious, is this some sort of broad band friction enhanced Helmholtz resonator?

I was actually planning on getting a piece of 12"x12"x1/16" natural rubber gasket material and cutting out the central cross section with the laser cutter and then using it in the center, with some clamps to seal the gasket, in order to allow me to check the tonal quality of the resonator cavity.

Based on a couple of spots I am seeing on the outside, I am concerned about the possibility of internal leaks, so I am trying to plan my next steps if my fears are confirmed.

The cracks should be thin, but there is no way of wiping excess sealer from the surface in the narrow resonator gaps. Hence the requirement that the sealer be a thin liquid. My thought was to spin the chamber as the resonator cures to prevent pooling. Also it wouldn't require full immersion, as the bottom on one half is completely sealed with the exception of the central 2" hole which I am far less concerned about and is isolated from the rest of the cavity on that side. The other half has a similar 2" hole that a fluid path would lead to, so I would have to seal it off (rubber and clamp style depending on the sealer used). Then the liquid can just be poured into the cavities and left to soak in.

I will take a picture of the exterior flaw that has me concerned so you have an idea of what could have gone wrong in the middle.

Out of curiosity, is cactus juice a hard finish or a soft finish?
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The resonator is a single air column folded back over itself repeatedly. Pinhole leaks between the folds can potentially affect the first fundamental frequency of the resonator, as well as create sideband peaks (which could be interesting). However, this resonator is not used in isolation, and needs to be tuned an octave down from another piece that gets plugged into the resonator.
Wow, hard to imagine the equations or testing you did to account for the friction of such narrow passages and the differences between each ring. It looks like the area of each ring is the same, but friction will be different.
Keep us posted. Sounds like quite a project. I would be quite curious of the Q.

Is each end of a slot radiused? Not sure a pinhole would really hurt it, but only you know for sure.

As this is not being used to mechanically resonate, then flushing it with poly makes sense to me. Increased rigidity probably is a good thing.
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Yes, the cross sectional area of each ring arc triplet is the same as the central circle. I am not to concerned about friction as the chamber is intended to have a turbulent flow rather than laminar flow anyway, and added back pressure is actually a really good thing.

By poly, i am assuming you mean polyeurathane. Is that correct?
Sealing this device has my head spinning. :)

Before I retired, much of my life was spent working on what kind of adhesive/polymer/coating material to use, and how to apply it in high volume with custom mfg machines; all of which I formulated, designed and patented. I've dipped, sprayed, swirled, dispensed, transferred, impregnated, cast, printed,and injected; just about every resin or liquid polymer system on planet.
IE #IAMAKLUTZ, and not an expert in many topics, except maybe this one? :)

IMHO - the 1st challenge is determining the kind of final properties you want or need?

My SWAG based on available information:

- Want a hard/rigid finish to prevent absorbing/changing the deflected sound
: this removes 'softer' polymer systems like polyurethane, polyester, or silicone.

- For solvent based system; need medium to low solids ratio to ensure penetration into cracks. Yet, it needs enough solids to fill cracks after evaporation. This can be a knife edge type of problem to find right solvent ratio?

: Solids ratio coupled with need for hard material, narrows the choices to: Shellac, Nitrocellulose lacquer (blend of nitrocellulose, tree rosin, and shellac), or Varnish (Old school instrument/electrical varnish, not modern varnish/poly blend wood finishes). Have listed these from lowest solids/thinnest coating to thickest. All these are readily available, though you will need to find an industrial coatings supplier to old school varnish and lacquer.

- For a 2 part, or heat cure system; need a low viscosity system with low wetting angle to ensure penetration.
: Use of curing mechanism enlarges number polymer options, but also shrinks options due low viscosity requirement. Primary choices are: Low viscosity epoxy resin, Heat cure 2 part acrylic resin (like cactus juice or Alumilite), 2 part vinyl sealer (used commercially for grain sealing furniture and instruments), or conventional 2 part conversion varnish top coat.

Epoxy resins might be a great choice, but need to find the one lowest viscosity that is also very hard. Many laminating resins are designed to be more flexible to reduce cracking with carbon fiber structures. The harder systems tend to use short chain length curatives, which have short pot life; and will make application hard for large pot mass immersion. The lowest viscosity systems use long chain reactive diluent to lower viscosity, but these tend to be soft. Furthermore epoxies don't dilute very well with solvents to create lower viscosity/lower solids; as it tends to create space between cure polymer molecules and makes for softer coating. While not impossible, would take a lot work to find the 'right' epoxy for this.

Challenge with vinyl & conversion varnish options is they are normally sprayed in thin films, and your resonator needs to be immersed, or heavy coated and then excess removed (thanks to inaccessible cavities)? Pretty sure they will work with immersion, as long you find a 6-8 hour pot life material.

- Need for turbulent flow also adds a twist? This forces use of much lower viscosity and/or thinner film build. Last thing you want is high solids coating, filling in all the rough edges you worked so hard to produce?
: This twist makes me think the best choice would be solvent based coating, or either of the 2 part coating systems (vinyl/varnish) which can have solids content adjusted with solvents.

- Considering need to fill in a 1/8 w x 6 mil h gap:
: Would want to use a 2 part system with as little solvent as possible.

My primary recommendation would be a 2 part conversion varnish.
Have used Mohawk post catalyzed conversion varnish, it should work in this bizarre application? They make a CV sealer with some vinyl in it to make sanding easier. The sealer has higher solids with low viscosity, and might reduce need for more solvent? ML Campbell makes Krystal product that is similar stuff. Had issues with Sherwin Williams CV lifting on re-coats, but for a one coat/dip project it could work too.
These are designed to be used with 5-6mil film thickness, and should have no issue curing in your gaps, nor will it crack/craze. Suggest you will want to add a low temp (120-140°F) bake for 4-6 hours after the coating has normally surface cured to ensure center of the gaps are fully cured. (must wait for normal cure to complete!)
I have never used a 2 part CV in this same manner, so highly recommend you take some scrap wood pieces, stack/glue them same way, and test your material/process before committing the final piece.

Secondary choice would be a heat cure 2 part acrylic resin.
Have never used Cactus Juice, only used Alumilite couple times, so I can't suggest one directly. All the other acrylic casting systems I used where mil-spec certified, and are not affordable to anyone except the military or aviation industries. I reluctantly suggest this method as these systems tend to have higher viscosity than coating, and might required pressure pot or vacuum pot to force the resin into gaps. Only with testing will you know what is needed.

One last polymer tip:
2 part systems tend to be moisture sensitive. Meaning trapped moisture will delay/interfere with catalyst and curing. You want the assembly to be dry before coating. Acrylics are worst and required less than 5% moisture content. Even with CV, moisture slows things down. Suggest you bake the wood for 8-10 hours in a low temp oven, (140°-160° maximum to reduce wood movement) before you apply the resins. Remove the unit from oven, let it cool 1-3 minutes and immediately coat before it picks up surface moisture. If you live in low humidity region, can be more relaxed about dwell time between oven - resin, maybe 10-15 minutes.

You are lucky. I normally charge a consulting fee for this kind of work. And would be glad to be hired by you to do the test and evaluation work on potential candidates for a solution. HaHa
But I also believe in paying it forward when it comes to hobby work, so no need to send me a case of TP or anything else cool for sharing the above information. :)

Best Luck.
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Thank you for your very detailed and well thought out response. The rough internal edges are not critical, the turbulent flow is produced by the lips rather than the surface. A smooth surface finish is not undesirable as long as it is hard. Does that open up better options, or should I still be moving towards two part conversation varnish?
Thank you for your very detailed and well thought out response. The rough internal edges are not critical, the turbulent flow is produced by the lips rather than the surface. A smooth surface finish is not undesirable as long as it is hard. Does that open up better options, or should I still be moving towards two part conversation varnish?
- Silvanus
Your welcome, and Hmm.

The width of gaps needing to be sealed, leads me to believe a 2 part or heat cure system would be better than single component room temp cure product? Not that a single component won't possibly work, just that it probably take weeks to cure inside the gap unless it was applied a small amount at time?

Sure would be whole lot easier if you knew exact areas that needed sealed?
Aviation industry has used several coats of nitrocellulose lacquer to seal wood/fabric for 100 years. Just keep it away from flames. :) Or could brush on some 30 min epoxy where needed and be done?
That was primary reason I bothered to suggest testing for leaks.

If can live with thicker film build and smoother walls;
- epoxy becomes more viable option.
Thinking about gap size, and pot life needed for large mass; would want to use casting epoxy, similar that used on river tables? Alumilite is one brand, but is most expensive stuff in market. Although their website is great for getting educated on casting resin materials. :) I formulate my own recipes, so not much help with picking one of the commercial casting systems. Google can find you many places selling casting epoxy resin.

- 2-part polyurethane might work too.
2K Poly finishes have some nasty curatives, requiring full body PPE, and are not something recommended for average home shop? The PPE is not cheap either. Probably not a good choice? Don't even think about using a room temp oil based polyurethane finish, as they require oxygen and moisture to cure. The center of gaps would take months to solidify.

- Another option would be a heat cured polyester resin. They are used to improve dielectric isolation on electric motors/transformers. They make polyester resin in various viscosity to control film build, and fill wire gaps. Would want one made for large OD wire and big gaps. Polyester is cheaper than epoxy or Polyurethane. IDK which on to use right now? Would have to dig around to find recommendation as polyester systems I've used are only used in industrial setting by the 55 gal drum? While safer than a 2K polyurethane, the waste disposal aspect of uncured material makes it not used in home shop? Have to oven cure left over material to dispose of as safe solid refuse, just like you heat cure the item when coated.

My best idea is figure how much sealing is needed first.
If only couple spots, then pretty simple brush on epoxy where needed.
Unless you want the entire structure to have uniform surface finish, they likely need to use a material for immersion. In that case, post catalyzed CV will have lower viscosity and can fill < 6mil gaps, epoxy will be better if you find even larger gaps.

Hope this helps more than it confuses. Materials selection encompasses so many criteria.

Remember, what ever you use, test on scraps first to get comfortable working with the new material.

Best Luck!
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Poly, yes.

I was not thinking about continuous flow, but sound. My reference is based on loudspeaker port turbulence. It could change the tuning quite a bit. Your design looks somewhat like what we called a transmission line. It was not uncommon to taper them as they folded. Not as critical as the impedance match at the throat. I found them too touchy and went back to my prefered low Q sealed boxes for woofers and various alignments around C4 for mid-base. Thinking about it, a wind instrument is "nothing" but a transmission line you introduce leakes into to change it's resonance. "Nothing" being the oversimplified detail. There is nothing simple about even a pennywhistle.

Anyway, that is why your project is so interesting.


Yes, the cross sectional area of each ring arc triplet is the same as the central circle. I am not to concerned about friction as the chamber is intended to have a turbulent flow rather than laminar flow anyway, and added back pressure is actually a really good thing.

By poly, i am assuming you mean polyeurathane. Is that correct?

- Silvanus
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