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banjonz - Posted - 01/09/2025: 15:49:26
I posted earlier about repairing a neck crack banjohangout.org/topic/401203/#5072110.
I did some searching online for crack filling and found a YT vid that used a filler over the repaired crack. I could not find the vid again but am wondering if the filler was a two pot/2 part one, something like a builders filler or automotive one? After sanding the repair smooth, I used my airbrush to spray black automotive lacquer over it. It worked well but the repaired crack line was visible. At this point I didn't put a second coat over it. I used some 2 part builders filler I had here and managed to mostly fill the crack.
My question is, is there are specific type of filler that luthiers use to do this sort of job? Remember, being in new Zealand we don't have access to the range of products you do in the US.
banjoT1 - Posted - 01/09/2025: 21:06:58
If you've color-matched the break line, then my opinion would be to stop there for a moment to think of how deep you want this repair to suck you into the consequences of over-fussing where you inadvertently turn a little deal into a big deal (meaning: "i shoulda known when to stop").
If what you're seeing is an almost imperceptible' groove' or indentation that reads as the break line then, if you did a great job in gluing, I would suggest you rely only on multiple spay layers that, when done right, will serve as the only filler you need.
Since I have no photo to peek at regarding this crack line, all I can offer is, spay, level out, spray, level out.......repeat as necessary until the finish has filled the crack. This recipe assumes you've made the necessary color-matching, etc.
In all likelihood, whatever "filler" you might introduce, may eventually "telegraph" - in a very big way - that you put some oddball 'stuff' in there, and it will loudly broadcast not only a repair but a rookie repair to boot. (IMO of course.)
Bill Rogers - Posted - 01/09/2025: 22:02:07
Trying to make the crack line disappear is a fool’s errand. Neither necessary nor remotely worth the (likely-to-fail) effort.
desert rose - Posted - 01/10/2025: 06:55:00
I agree with you Bill, the job at hand is structural integrity. Visual repair is reserved for air brushing and coloring the finish part of the repair.
Quickstep192 - Posted - 01/11/2025: 06:11:02
I usually use thin epoxy to fill cracks. I warm it to make it thinner yet. (MAS Low-Vis Epoxy)
But, there are two tricks that I’ve used to help get as invisible a repair as possible.
I fill a bit above the surface. I often mask with blue tape around the repair and that is usually high enough above the service for trick #2. Instead of sanding all the fill level with the adjoining wood, feather the edges, leaving the fill over the crack a little bit high.
Trick #1 is to let the filler fully, fully cure (like a week). It’s tempting to sand as soon as the filler hardens, but I’ve found that many fillers continue to shrink a tiny bit as they fully cure.
I discovered this when filling cracks with epoxy. I’d carefully overfill a crack, then sand the next day, followed by finish. A week later, I could see a slight concavity where the filler was. It was very small, but painfully obvious to a critical eye under a gloss finish.
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