Puzzled: Gap Appeared on Rosette During Finishing

I'm about half way through lacquering one of my ukes (6th coat) and noticed a very small gap appearing in the rosette channel. I'm puzzled as to why this appeared, but I remember a forum member (forget who) saying something like: "There will be hell to pay when it comes to finishing out the uke if you don't do..(something, something) because gaps can appear." I can't find the thread, but the words are echoing in my mind.

The channels were treated with a 1:2 cut of shellac before gluing with CA glue. There were no gaps after gluing and sanding. Absolutely flush and perfect fit. What is happening here?

My theories:

1) I'm using a water-based acrylate lacquer and during pore filling, the wood swelled and shrank.

2) The shellac has dissolved away a tiny bit leaving a gap

3) Insufficient CA glue fill which has.... ???

4) ?

This is hardly visible at all and I think I can fix it by flooding the gap with lacquer, but I'm puzzled as to what is going on here. I'm sure you pros can help.

Thanks for any input. Pictures below. Interestingly, the gap has only appeared in one place and the rest of the rosette is flush and tight (second picture)

it looks like you may have missed on the pore filling. May have been filled with dust and you didn't notice it.

Yeah, but it was tight before pore filling. My new theory: Insufficient CA filling originally and the gap was hidden by the dust after sand-out. The dust has now been washed away during pore fill and lacquering and the gap appeared. But I would swear there was no gap there before... Anyway, this is something only me and the rest of you luthier guys would ever notice. But I don't like it. Oh also, the rosette is suddenly proud just a tidge (maybe 1/256th inch) at the spot, Hmmmm...

could have been a bubble in the glue

That looks like a type of Spruce? Why are you pore filling Spruce?
Anyway, the wood or Shellac did not shrink. I strongly suspect the gap was there from the outset. If you had used something like HHG there's a chance that it would have swelled the end grain, making a tighter fit (but perhaps not perfect). CA or shellac won't do that. More CA won't hide a gap. It will fill it but it will still be seen as a gap.
Of course HHG brings it's own problems in respect to any Black dyed purfling/wood, which you need to be aware of.

This is not uncommon. Lacquer will show tiny gaps like this that are too small to notice during pore filling. LMI has a finishing article on its web site that addresses this issue. Don't flood the gap with lacquer. Instead, use a fine artist's paint brush to apply a tiny drop of lacquer to the gap, painting it on rather than drop filling. Repeat this at about 15 minute intervals until the lacquer is proud of the rosette. Remember, the lacquer will shrink as it gasses off. Let dry at least overnight and spot-sand flush fine wet-dry sandpaper. Good luck.

Trying to fill that with lacquer could easily be like opening Pandora's box. And, it will just shrink back in anyway, for months it will shrink, and it will never fill it so that it stays flush. Filling that with lacquer without wrecking the surrounding finish will take some skill, leaving zero trace even more so.
If that were my repair, I would fill it with medium CA glue, carefully, then scrape what I could back down as much as possible with a new razor blade with tape over all but the center of the cutting edge, enough to 'get' the glue fill. Since scraping CA is a mixed bag, as it likes to fracture, there would probably be some detail sanding with a small 'block', trying to focus on the high part of the glue. Then I would sand the fill, being careful not to go through the lacquer. 'Feather' sand the finish to eliminate all traces of the area being molested, then re-spray the top. Basically, go back to setting the rosette with CA, then refinish the top.

Acetone and other hot solvents can erode or at least weaken CA, so if that was ever really looking tight, there probably was a bubble in there, with a thin covering. Perhaps the lacquer melted it. Or perhaps, it was never really filled with anything but dust.

Totally sucks to find things like that in a finish, at the stage where you are at.

As Michael says, the CA will not hide the gap, the fill will look like place where the black line gets a little uneven, wider.

These gaps can be difficult to repair invisibly but it can be done if you've got the patience. The problem is that any fresh lacquer will "melt" all the previous coats. I have a jar of lacquer that I let evaporate to the consistency of thick maple syrup. Wearing my most powerful (10X) glasses I pick up the tiniest drop of lacquer with a pin or an A string. The idea is to fill only the gap, avoiding the surrounding area. Don't trap any air! After a day, look at the spot as see how much it has shrunk, you may have to repeat the application one or two more times. At this point I like to wait at least a week (longer is better) before scraping it flush with the surrounding finish, then sand and buff as usual. Use a safety razor blade scraper that you've created a burr on the edge and protect either side with tape. If you don't know what I'm talking about, ask and I'll try to post a pic.
If you try to hurry this process and/or are messy about it you will notice a depression ("halo") in the area sometime down the line. It may take a month or more to show up but it will eventually show.

EDIT: I was typing at the same time Chris was. Good advice there as well.

Moore Bettah Ukuleles said:

These gaps can be difficult to repair invisibly but it can be done if you've got the patience. The problem is that any fresh lacquer will "melt" all the previous coats. I have a jar of lacquer that I let evaporate to the consistency of thick maple syrup. Wearing my most powerful (10X) glasses I pick up the tiniest drop of lacquer with a pin or an A string. The idea is to fill only the gap, avoiding the surrounding area. Don't trap any air! After a day, look at the spot as see how much it has shrunk, you may have to repeat the application one or two more times. At this point I like to wait at least a week (longer is better) before scraping it flush with the surrounding finish, then sand and buff as usual. Use a safety razor blade scraper that you've created a burr on the edge and protect either side with tape. If you don't know what I'm talking about, ask and I'll try to post a pic.
If you try to hurry this process and/or are messy about it you will notice a depression ("halo") in the area sometime down the line. It may take a month or more to show up but it will eventually show.

EDIT: I was typing at the same time Chris was. Good advice there as well.

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I don't see how you can repair that invisibly. It's against Spruce, it will always show as a 'shadow', irrespective of how carefully done or indeed what you fill it with. The differing reflection will always give it away.

What Chris and Chuck said.

Also, CA onto lacquer can cause the 'halo' that Chuck mentioned. If you uses CA and it gets onto a finished lacquer surface, ( please note also, that I do not have a ton of experience with repairs like these because I try to get it right the first time round, and am okay at doing that) it is a good idea to let the piece sit for at least a couple of days, and maybe a bit more, I do not know. I have seen CA onto lacquer, causing problems, where I continued finishing the same day, and it took some 'chasing' to get the finish right again. If you use CA to fill that gap, let it cure for a while... a day? 2? I think the lacquer softens when CA touches it. It definitely needs to harden up again before continuing.

The easiest fix I can think of is to get some oil putty from the paint store, mix it to a spruce color, maybe one of the 'off the shelf' colors will work. Mix that with a little corn starch to stiffen it up a bit, then 'smear' some of that into the gap. Wipe it carefully with a clean rag, trying not to erode the putty, and also being extremely careful to remove any oil smearing on the surface, from the putty, then spray another coat or two of lacquer. It will indent slightly, but so will a lacquer fill. There will be no more gap.

Shellac 'burn sticks' are also an excellent way to repair something like that, and in just now remembering about it, that is probably what I would opt for. This involves a heat melted stick of colored shellac (?, never was exactly clear what that stuff is, it feels kind of like hard wax, or P-tex) While it is hot, it is pressed into the gap, then rubbed smooth. This is a professional repair that takes some experience, but it can be mixed to match the spruce color. It is possible for this repair to not indent. Burn in sticks can make for great repairs.

I've had some of those sticks in drawer for years but have been too chicken to give them a shot.

Michael N. said:

I don't see how you can repair that invisibly. It's against Spruce, it will always show as a 'shadow', irrespective of how carefully done or indeed what you fill it with. The differing reflection will always give it away.

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The repair may never be invisible but you can get close following the method I described. It's really a microscopic repair done very carefully. If the gap is caught early then it's a simple matter of drop filling and spraying subsequent coats over it. That type of repair will be non detectable.

Burn in sticks, putty, lacquer. . . doesn't matter what you use, it will always show. You can never match the reflection of the Spruce and it's the mismatch in reflection that you will see. Trust me on this. I'm afraid Spruce is very unforgiving when it comes to hiding flaws. The closest I've come is to mix Lycopodium powder (old Violin makers trick) with a touch of Lithopone and glue. Play around with them and you will get a very close colour match. You might get a bang on colour match but it will still show though. It's not the colour that is the problem. it's the way that the surface of the Spruce and the 'filler' reflect light differently. From one angle it may appear perfect, move your viewing angle slightly and it will suddenly appear. I know that it's not much of a gap but I guess it depends on how closely you look at these things. Very few Players will notice. Some makers may not but a lot will if they are looking at it critically.
You can use the Spruce off cuts from the soundboard and insert/graft little 'V's' but that's pretty advanced restoration work, it can also make matters much worse if you aren't experienced at that type of repair. The only other way to solve it would be to remove the inlay and very carefully enlarge the channel/rebate and put in Black lines next to that shell. Either that or reduce the width of that shell a little.

+1 on that..


Best is to build another uke and remember what happened on that last one..

It can certainly be saved and I certainly wouldn't be in a hurry to scrap it. Things like this happen, even to me and I've been making instruments for the best part of 30 years. The 'trick' is how you recover from these set little set backs and/or whether you simply leave it be and call it 'good enough'. I don't think I've ever made a technically perfect instrument and I very much doubt I'm going to start doing so now. Perfection probably isn't all it's cracked up to be anyway but as a maker you have to draw a line somewhere, otherwise a single instrument might take months of work to finish. Life is a bit short for that kind of thing IMO.

Moore Bettah Ukuleles said:

If the gap is caught early then it's a simple matter of drop filling and spraying subsequent coats over it. That type of repair will be non detectable.

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Totally agree with Chuck on this matter. As long as you are still in the finishing process, you can fill the gap with thick lacquer or CA, then sand and respray over the area and it should be invisible. Thick lacquer will still want to settle and it may take time till it stops crawling. CA cures fast so you don;t have to wait that long, but it MUST be totally dry and filled and sanded flat. I use thick CA and never use the spray accelerator with this kind of problem as this will cause it to bubble. This method does not work too well on an older instrument because of the aging lacquer and color difference.

I also agree that any kind of lac stick, putty or wood filler will show and mostly look awful.

BlackBearUkes said:

I also agree that any kind of lac stick, putty or wood filler will show and mostly look awful.

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Thank you all for your input. Really good stuff. What one has to keep in mind is that the pictures posted were taken in extreme macro close up with the most unflattering contrasting backlighting possible to show the flaw. In real light the thing is barely detectable. But still. It ain't right and has to be addressed.

So... I think I figured out what happened. The flaw appeared at the most vulnerable position of the rosette cut which to me is the 8 o'clock position where the spruce grain transitions on the left side. The hideous danger zone. Why it appeared flush was an illusion. The wood was really micro-mush if you will, and gave it up when the lacquer coats hit it. I don't think it was a CA bubble. So, I think Chucks advice is best and that's what I'm doing. So far so good at filling the canyon with lacquer by painting it in. However, there will always be a slight "shadow", but really, we are talking microns here. The one fix that nobody suggested that I think is a possibility is mixing a little abalone dust in a clear lacquer and delicately putting it in there with a magnifying loop would actually fix the problem. A delicate scrape and a little 800 grit. Hmmmm... Scratch that idea. I would probably just appear as a white line.

Anyway, this is an experimental uke and not a commercial product so nobody, and I mean nobody will ever notice. Except me. And you guys.

If it doesn't go right the first time it wont be rectified - rule #56

I hear ya on that one Pete... Also Murphy's 12th law: If you play with something long enough, eventually you will break it.