Official Luthiers Forum!

Wow - I love this forum. Posted a question this morning and have three thoughtful replies already. Thanks all!

I'm a novice luthier (finishing my fourth steel string) and I seem to consistently have trouble with the fit and finish of the rosettes on my guitars. I'm gluing them (usually a combo of purfling and abolone in conentric circles) and the fit looks quite good when the AR glue is dry and scraped, but after sealing and finishing the tops (using KTM 9 water based finish) the finished product shows a "trough" on the seam between the rosette and the spruce. I've ususally used a coat of shellac to seal the spruce and prevent chipout when routing the cavity for the rosette. Also have followed the six coats, sand then six coats of sprayed KTM 9, but still doesn't finish smooth. Thanks

Well depending on the type of purfling being used, PVA glue will swell wood or fish paper purfling about .002" or so. So if the channeling is properly sized to the shell width + purfling width if PVA glue (white or yellow wood glue) is used you should have a tight fit. I am probably having a brain fart right now but you said you use AR glue. This is not ringing a bell with me right now. Also you should use either 1# cut of wax free shellac or egg whites or even vinyl sealer to sand seal the entire top prior to applying top coats.

If your topcoats are shrinking back into seams then the seams are acting much like open pores. Some improvement on the fit of the channeling and or rosette component sis in order. Or improvement on your scraping technique. This combined with a good quality sanding seal should do the trick

KTM-9, and most waterbased lacquers, are notorious for shrinking back into binding and rosette seams, especially when using AR glue. A pretty compulsive attention to surface prep is required, and even then seams tend to show up after a while.

Possible things to try, beyond installing the rosette as well as you can-

-KTM-9 doesnt shrink in to seams as much when they are glued with hide glue, superglue or epoxy. Superglue works well for rosettes, if the channel is sealed with shellac first.

-Micheals suggestion of shooting some sanding sealer on first may be a good idea.

-Spend some more time with the shellac step, i like to french polish the top thinly, and then spray waterbased laq over that. I like the color of shellac, and french polishing works the shellac into the seams of the rosette and purflings, sealing it up nicely.

-If you are pore filling with an epoxy filler, you could try going over the top with epoxy, really getting into the rosette seams while avoiding a thick film on the top.

There are plenty of high high end guitars out there with sinky glue joints, so dont feel bad.

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Jordan Aceto
Ithaca, NY

I forgot an important one- the longer you wait before sanding KTM-9, the less it shrinks back into the glue joints.

I dont like to touch the instrument for a month with KTM-9, although it sure is hard to wait!

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Jordan Aceto
Ithaca, NY

AR = aliphatic resin

I'm not enough of a chemist to know why PVA (polyvinyl acetate, but you knew that) glues are sometimes called aliphatic resin, but there you have it.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

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