MIMF

This is a problem that has plagued me off and on for a few years and I'm not sure of the cause. After the final coat of about 12-15 thin coats of lacquer, I let the finish cure for at least two weeks. I then wet sand until I'm satisfied and then buff out the instrument.

After about a week to ten days, a fine line sometimes appears in the top right on the top center joint from the soundhole to the tail block area. I have resanded and buffed out again only to have it reappear a week later. This happens before the instrument has been strung up so tension is not the cause. I build ukuleles so I usually use Koa for the tops but it has happened with Cedar also. Sometimes it doesn't happen at all.

I use original Titebond for the joint and have sometimes used thick CA. It has happened to both of those glues. I believe my glue joints are good. It has happened with waterbased lacquer as well as Behlen's nitro. I don't use a sealer. I use West System's epoxy as grain filler, except on the Cedar tops. My workshop is about 50% humidity but the instruments are out of the shop and in slightly higher humidity by the time it happens.

What the heck is happening? Is it the glue? Lack of sealer? Joint? Humidity changes? Any insight into this would be welcome.

Steve

I suspect you are right about the glue being the problem. The PVA glue will absorb moisture and swell some. I don't know why the CA would do this however.

I use Hot hide glue for the structural joints in the guitars, and I have not had this occur. I have seen this with PVA glue in my other wood working projects.

Hi Steven and welcome to the forum! Interesting that it happens under no tension- have you ever noticed the "cracks" occurring before finishing? How thin is your top?

Maybe the synthetic glues are reacting with something in the lacquer? I use hot hide glue these days, and have not had that problem since I switched.

Alan Carruth / Luthier

I have seen other reports of a raised glue line showing up after using water-based finished.

Thanks for all the replies. I've been thinking about switching to hot hide glue for some time now so I guess it's time.
The line appears to be sunken rather than raised so I'm assuming that the glue is shrinking or is reacting with the lacquer as Alan suggested. I don't use old Titebond but I'm wondering if it has a shorter shelf life than I thought and is not curing like it should. I usually get a new bottle after 6 months or so.

Charlie, my tops vary in thickness depending on how they flex but are usually in the 1/16" +/- area. It only has happened after finish is applied as far as I can tell.

Barry, I started off with waterbased lacquer but switched to nitro hoping that would stop this but it still happens with nitro.

Looks like I'm switching to HHG.

Steve

I've run across two common reasons for raised glue lines:

Changes in wood moisture content: If wood is not sufficiently dried, or is moved into a drier environment than where it was stored, it can shrink, in width as well as in thickness. A PVA or aliphatic glue line will allow the wood to "creep" as it shrinks but the glue line will stand proud. Half a thousandth of an inch is more than enough to show up.

On glued-up panels, it's possible that two pieces of wood with different moisture content (could be as little as 1 percentage point) are jointed and glued together with parallel grain. The wetter wood can dry into equilibrium with the neighboring piece. In doing so, a step will be created at the joint.

I would suspect the first cause, in this case. The wood is probably stored and/or glued up in one environment, and then brought in to an environment with a lower relative humidity. As it slowly loses moisture and shrinks, the glue line seems to rise proud of the surface. This is the time of year for that to happen. It's possible, too, that the wood just hadn't reached equilibrium moisture content with its environment before it was worked. Same result.

Wood finishes can slow moisture transfer, but won't stop it. Wood has to have the right moisture content when it is worked, to match the normal humidity of the environment where it will "live". Better too dry than too wet, IME.

Hi Steven,

One possible that you didn't mention was heat. How warm does the wood get when buffing? I mention this because you can bake out moisture in 1/16" wood pretty quickly, and the wood will later swell as it returns to it's pre-baked moisture content. What looks like glue lines sinking may be wood swelling as it normalizes.

Good luck...

Thanks Dick and Michael for your insight. I stripped the top lacquer from an affected instrument, sanded, and sprayed on some Behlen's vinyl sealer. I resprayed lacquer today so I'll see if this is the solution or not. Humidity could also be the culprit but I store my wood in a humidity controlled cabinet and my workshop averages about 45-50%. Dick, I'm very conscious about heat when buffing but I'll see if I can back off a bit in that area.
All of my future instruments will have HHG for the top and back joints so hopefully I won't have to deal with this anymore.

Thanks to all,
Steve

This is a problem on solidbody electrics sometimes as well.

It doesnt raise the glue line, it actually sucks the finish into the glue line. If it raised the finish at the line, it wouldnt matter at all- wet sanding and buffing would knock it down.

I dont know about now, but Charvel was glueing maple veneer tops and backs on their solid color painted bodies to eliminate the problem. The glue line would be in the radiused edge which would be difficult to spot instead of the front or back of the instrument.

It happened to 2? of my builds both glued with titebond origional, sealed with west systems epoxy and finished in automotive 2k.

Currently, when I'm ready to seal, I flood all the glue joints with thin CA then seal the entire guitar with west systems.

The CA seems to help a lot, but I did get one small line on one- perhaps it didnt flow in very well, or I level sanded too deep in that area.

With the 2k automotive, the longer you wait before sanding and buffing the less likely it is to appear, since it will pull the finish into the glue line before sanding.