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I am building my first acoustic guitar based on plans from a book and so far I have been very pleased with the results. The problem is that I have been working on the finish for about the past month with unsatisfactory results.

I am using the Cardinal finishing products (sealer, pore filler, and nitro lacquer). The guitar already has the neck attached which I have been told is a bad way to build a guitar and you should wait until after the finishing process to attach the neck (too late now). The book instructions guide you to attach the neck before finishing so that is what I did.

Here is the problem: Everything was going well and the finish was looking really good until I noticed tiny bubbles in the neck to body joist. I have been spraying 3 coats a day with an hour cure time between coats. I then level sand the next day and spray three more coats, etc. to build up the lacquer. Everything was building up just fine and the neck to body joint had a small build-up of lacquer with no bubbles and looked great until that third round of finishing. Then it all went downhill from there and tiny bubbles were everywhere in that neck joint.

I tried to sand the bubbles out but it was nearly impossible in such a tight joint. I had to finally remove all of that finish in that joint with lacquer thinner (on a Q-tip) and re-seal, and re-pore fill that area. Got is all sanded out and built up a couple of layers of lacquer and it looked really good again. Then I sprayed a couple more coats and it bubbled again! That second round of spraying after removing the first round of bubbles I was extremely careful to spray very light coats in that area (as I thought it was building up too much too fast the first time and caused those bubbles). No luck, it still bubbled. The rest of the guitar looks excellent. I am wondering what to do next? I can strip it all away in that neck-body joint and start over again but I am worried the same thing will happen.

Anyone have this issue before? Any ideas for the cause and what I can do to fix it?

Thanks!!

Sounds like some contamination/incompatibility in that area. A picture of said area
would help get a more accurate diagnosis. I've never used Cardinal but I doubt that
your problem has any thing to do with the specific brand. Post a pic for us to gawk.
Ken

Here is a pic of the bubbles in the joint... Tough to see in a photo but you get the idea.

First here is a copy of my spray finish problem solving chart. http://howardguitars.blogspot.com/2015/ ... rt_18.html

I think the problem is air entrapment in the joint itself. Try dropping a bit of retarder on just the joint to soften up the lacquer and let the air work out. Be careful to only soften as much lacquer as needed so you have a smaller area to oversparay at the end.

And there is nothing wrong with attaching necks before finish, Gibson and others do it that way......

_________________
Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

Thanks Brian! I really appreciate your time. I will check out the flow chart and try the retarder trick. Any particular brand of retarder you recommend?

Glad to hear others do attach the neck prior to finishing. To me, that seemed like the only way to do it until I talked to a few local luthiers and they were very surprised that I did this.

Air bubbles in this sharp corner between the heel and side, as well as the corner between the fingerboard and top have been my biggest headaches while finishing guitars.

When I was building in New Zealand, I ran into the problem with a pre-catalysed polyester from an Australasian company that offered a variety of thinners. Going for a slightly slower thinner and being careful not to direct spray directly into the corner eliminated the problem -- presumably a thin film with more time for air to escape.

Now, in Canada, I'm using Mohawk instrument nitro and the problem has returned. Unfortunately, Mohawk doesn't seem to offer an array of thinner speeds, so I'll have to experiment with retarding additives to get the same effect.

As Brian suggested, running a thinner or retarder in the corner can allow the air to escape, but it is really fiddly and you'll be facing some tricky touch up. Obviously best the avoid the problem and I wish I could offer a guaranteed way!


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It sounds like with your previous success it is definitely an issue of the lacquer curing too quickly in that joint. Seems like the trick will be finding the fine line of how much retarder to use to allow the perfect cure time to avoid bubbles without using too much.

I found another person that has seen this issue many times and says the reason is because where the neck meets the body, no matter how perfect the joint is, is still an open area with tiny air gaps. Their remedy is to use pore filler and or super glue along that joint to completely seal it off so no air is in that joint. I don't know if I would use super glue with the worry of getting different colors at the glue to bare wood transition once sprayed with lacquer.

I assume the trick with the pore filler would be (in my case) to use the mahogany color and fill that joint without getting any on the rosewood. Seems a little tricky and nearly impossible. I am going to try the trick of dripping in the retarder just along the joint line and see if that breaks it up enough to get rid of the bubbles.

If your not on any kind of deadline, it might be a good idea to put it away for a couple of weeks and come back later