Ukulele finishes

BuzW

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Mar 17, 2022
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I’ve made nearly 50 ukuleles but I still sometimes struggle with the finish on some kinds of wood. Woods like Milo, and Koa are of particular concern. I seal the wood wit 3 or more coats of UV resin, then spray between 10 and 15 coats of instrument lacquer on, sanding lightly between coats. After about a week of drying and hardening, I wet sand from 400 grit to 2500 grit. The I buff it to a brilliant shine. Okay, the problem arises several weeks after buffing. The finish seems to develop tiny bumps on most of the surfaces. It never seems to happen on the neck which is made of African Mahogany. I’m thinking is could be some of the oils or resins in the wood that are working their way to the surface through all of the layers of finish. Any thoughts or suggestions anyone?
 

Paul1973UK

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May 1, 2022
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I'm no expert, but if your concerned about the woods oils/resins seeping through the lacquer, maybe try a different sealer? Shellac makes a good sealer and is compatible with all finishes, maybe try making up a 2lb cut of shellac (120g of shellac flakes to 500ml isopropyl alcohol) and using it as your sealer coat instead of (or before) the UV resin, it can be brushed, sprayed, or padded on as you would when doing a French polish. It's cheap enough for you to give it a try on some scrap wood and compare it to your usual method... plus is doesn't take a week to dry and harden so won't lengthen your production times by much.
 

BuzW

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Mar 17, 2022
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I'm no expert, but if your concerned about the woods oils/resins seeping through the lacquer, maybe try a different sealer? Shellac makes a good sealer and is compatible with all finishes, maybe try making up a 2lb cut of shellac (120g of shellac flakes to 500ml isopropyl alcohol) and using it as your sealer coat instead of (or before) the UV resin, it can be brushed, sprayed, or padded on as you would when doing a French polish. It's cheap enough for you to give it a try on some scrap wood and compare it to your usual method... plus is doesn't take a week to dry and harden so won't lengthen your production times by much.
Thanks, I will try that, but I doubt if it would do a better job at sealing than UV resin. It is worth a try though
 

ProfChris

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Jul 17, 2009
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Three possibilities:

1. Nitro lacquer shrinks for much longer than a week. Normal practice for those seeking a gloss finish seems to be to wait a month before wet sanding level. Otherwise, the finish shrinks back over a few weeks into any unfilled pores. I'm too impatient (and incompetent) to go for full gloss, but I still leave nitro for 2 or 3 weeks before sanding.

2. It might be that the nitro is reacting with the resin to produce the bumps, if your resin filler is truly flat. You could test this by putting a coat of resin on something already flat, like melamine faced MDF/chipboard, then spraying your nitro on top to see if the same happens. If so, it's a matter of changing one or other component until you find a compatible combination by trial and error. Alternatively, a coat of shellac between the resin and nitro might be enough of a barrier - again, it's just trial and error.

3. Some kind of contamination of the surface from (or after) your levelling of the resin filler - a thorough clean with naphtha or acetone before the nitro might deal with that.

It might be oils seeping through, but I've never experienced that with koa. Again, a shellac barrier coat might prevent seepage.

Sounds like you have a series of test pieces to prepare!
 

BuzW

New member
Joined
Mar 17, 2022
Messages
3
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Three possibilities:

1. Nitro lacquer shrinks for much longer than a week. Normal practice for those seeking a gloss finish seems to be to wait a month before wet sanding level. Otherwise, the finish shrinks back over a few weeks into any unfilled pores. I'm too impatient (and incompetent) to go for full gloss, but I still leave nitro for 2 or 3 weeks before sanding.

2. It might be that the nitro is reacting with the resin to produce the bumps, if your resin filler is truly flat. You could test this by putting a coat of resin on something already flat, like melamine faced MDF/chipboard, then spraying your nitro on top to see if the same happens. If so, it's a matter of changing one or other component until you find a compatible combination by trial and error. Alternatively, a coat of shellac between the resin and nitro might be enough of a barrier - again, it's just trial and error.

3. Some kind of contamination of the surface from (or after) your levelling of the resin filler - a thorough clean with naphtha or acetone before the nitro might deal with that.

It might be oils seeping through, but I've never experienced that with koa. Again, a shellac barrier coat might prevent seepage.

Sounds like you have a series of test pieces to prepare!
What puzzles me the most, and leads me to believe that the oils in the wood are the problem is that the problem never occurs on the neck which is made from African Mahogany