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Crack fillers, and when to use it.


Kallie
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Kallie,  I use a recipe I got from Hans Nebel that works good.  It's basically shellac, sandarac and copal in equal proportions.  I let it thicken with time.  As far as when to fill, I mainly use it when I have a "valley" after gluing a crack.  Deft has commonly been used and very easily available at the moment as it's basically nitrocellulose sanding sealer.....kinda of.  Word on the streets is that deft shrinks a little over time.  Just use it to bring up to surface level.  Sometimes I use it for gaps in places and then touch it up because it's reversible.  jeff

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Crack fillers, and when to use it.

...and when do you use it? To what degree must the violin be cracked before it becomes necessary, or how big must the crack be?

Hi Kallie,

 

As for when to use it, generally after:

 

• the crack has been thoroughly cleaned

• the crack has been brought to level and test-clamped without glue

• the crack has been glued and clamped

• any necessary cleats or excavated patches have been fitted and glued (from inside the instrument)

 

Then, the crack filler is used basically as a cosmetic repair, and also to protect any exposed wood at the glue line or where varnish has chipped along the crack.

 

Perhaps you knew all of that, but it was possible to read in your question an implication that crack filler is shoved into cracks to "fix" them, which is not the case.  My apologies if it seems that I am lecturing you, but even if you knew all of that, it is possible that someone reading this thread might get that impression, and I didn't want to leave these things unspoken (er, unwritten).

 

A place where one might use crack filler without repairing a crack first is when there really isn't a crack to start with.  For instance, if a steel string has scratched an instrument through the finish and into the wood a bit without actually cracking the wood (more of a very narrow gouge that is not following the grain of the wood), then crack filler can be used as an effective touch-up material. Think of it as cosmetic rather than structural.

 

I am quite sure that there are many more experienced and skilled repair persons than I am here, so, if any of them want to weigh in, that might be helpful.

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Hi Kallie,

 

As for when to use it, generally after:

 

• the crack has been thoroughly cleaned

• the crack has been brought to level and test-clamped without glue

• the crack has been glued and clamped

• any necessary cleats or excavated patches have been fitted and glued (from inside the instrument)

 

Then, the crack filler is used basically as a cosmetic repair, and also to protect any exposed wood at the glue line or where varnish has chipped along the crack.

 

Perhaps you knew all of that, but it was possible to read in your question an implication that crack filler is shoved into cracks to "fix" them, which is not the case.  My apologies if it seems that I am lecturing you, but even if you knew all of that, it is possible that someone reading this thread might get that impression, and I didn't want to leave these things unspoken (er, unwritten).

 

A place where one might use crack filler without repairing a crack first is when there really isn't a crack to start with.  For instance, if a steel string has scratched an instrument through the finish and into the wood a bit without actually cracking the wood (more of a very narrow gouge that is not following the grain of the wood), then crack filler can be used as an effective touch-up material. Think of it as cosmetic rather than structural.

 

I am quite sure that there are many more experienced and skilled repair persons than I am here, so, if any of them want to weigh in, that might be helpful.

 

Thank you. Perfect explanation. Untill now, Ive basicly only had to repair hairline cracks. So filler was never needed. But a violin Im currently working on has a few gaps and small channels of wood missing, so even with the crack being closed, something more is needed.

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. Untill now, Ive basicly only had to repair hairline cracks. So filler was never needed. 

Kallie, even with hairline cracks, you will need to fill to completely hide that crack.  You will usually fill (not with the typical crack filler) with shellac.  I use the viscosity of what I use in typical touchup work. Dilute it until is will just suck down into the varnish crack, as slight as it might be.   I make a very fine bead to fill the crack (tiny, tiny).  I use a magnifier lamp (makes a big difference).  Even in a hairline crack, the varnish is disturbed and cracked, this works well for filling, leveling and then finishing it off flush (VERY light french polish, or just a alcohol, shellac based polish will hide it completely.....usually :rolleyes:   jeff

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Kallie, even with hairline cracks, you will need to fill to completely hide that crack.  You will usually fill (not with the typical crack filler) with shellac.  I use the viscosity of what I use in typical touchup work. Dilute it until is will just suck down into the varnish crack, as slight as it might be.   I make a very fine bead to fill the crack (tiny, tiny).  I use a magnifier lamp (makes a big difference).  Even in a hairline crack, the varnish is disturbed and cracked, this works well for filling, leveling and then finishing it off flush (VERY light french polish, or just a alcohol, shellac based polish will hide it completely.....usually :rolleyes:   jeff

 

Excellent, thank you very much for that description. Is there anything specific needed to dilute the shellac? Or will plain alcohol work?

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