Official Luthiers Forum!

Howdy all, I'm an amateur just starting out on my guitarmaking journey and I was hoping for some advice and insight into a problem I've encountered.

This is a problem I've encountered on my third build, an ambrosia maple/engelmann slope-shouldered dreadnought w/ ebony binding.

I had lacquered the body with a brush-on nitro lacquer (minwax) and was wet sanding to level it out. Things seemed to be going well and I had progressed to 1200 grit wet and dry paper when I began to notice some strange discolouration appearing in the rosette and purfling (see pic below). Both made use of the same bwb fiber purfling. I have used this same purfling before with no problems although my previous attempts at finishing were varnishes that did not require wet sanding.



At first I assumed I had sanded through, however lightly scraping above the discolouration revealed that there was still lacquer between me and the purf. Recoating with lacquer and levelling again didn't improve things at all. It looks to me like something, perhaps the water I'm using for wet sanding, has somehow wicked under the finish (perhaps though a tiny sandthrough?) and damaged the black fiber in the purfling. That's just my uneducated guess though.

At this point I plan to solve the issue by re-topping the guitar as this problem is both in the rosette and at several places in the soundboard purfling. Is that the best way forward?

In the meantime I really want to understand what went wrong so I can avoid the problem in future - it is quite a frustrating mistake to make so close to the finish line.

Any ideas on what caused this problem and how I can go about preventing it in future?

This forum is a superb resource for beginners like me and although I haven't posted much I'd like to thank all the contributors for the great information I've already picked up just lurking around and reading threads.

Josh



Sent from my iPhone using Tapatalk

looks to me like the laquer didn't want to adhere to the purfling for some reason. maybe the purfling was oily, or still gassing out something while it was laquered...?
i'd be inclined to stick the tip of an exacto or a pin in to it and see if i could pry off a flake; if so then it is likely air under the finish. perhaps you could chip out/route over the affected areas and try to drip fill some thin laquer in to the depressions with hopes that it would adhere the second time around

That could be possible, although these discolorations had not appeared during lacquering, during the three-week curing time, or even during wetsanding through two previous grits (800 and 1000). That's what has me confused - they appeared right at the end of the wet-sanding process. Prior to that everything looked nice. But you could be right - perhaps whatever reaction was occurring just had not reared its head until that point.

Recoating won't help unless you punch into the bubble. It definitely looks like finish delam. Not good news, but better than a retop.

There's folks here with much more finish experience to give you the fix. But don't panic...

That looks like glue under the finish to me.

Yep, looks like glue to me as well. Often you don't really see it until you start to polish out the finish.

_________________
Brian

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

https://www.howardguitarsdelaware.com/

I must concur, looks like glue.

Looks like off white from filling with CA glue??

Looks like it just follows the black.................??? Think that raises a question.
Tom

_________________
A person who has never made a mistake has never made anything!!!

Thanks for the replies everyone. The purfling were all glued with titebond. I didn't use any CA to fill them, but I did use CA/sawdust on the back and sides to fill wormholes in the ambrosia maple. So perhaps I inadvertently contaminated them by touching the purfs when I had CA residue on my fingers?

I did all the filling before final sanding, so I would have thought that sanding up to 240 would have cleaned any foreign substances off the purfs. Gave the while box a wipe with naphtha as well before I started lacquering. Any other steps I can take in future to avoid this situation?

Thanks again for the advice, everyone!


Sent from my iPhone using Tapatalk

That's not CA showing through, that's the Tightbond. The purfs weren't fully sanded before finishing. A wipe down with naptha before finishing will usually show missed glue so you can address the problem before you cause yourself a bunch of extra work.
All is not lost, you get the opportunity to learn how to strip and refinish a guitar now.

Best of luck.

Steve

Ok great, thanks for the tips. I'll be more thorough in my finish prep in future. It's fun journey, everyday I gain a better understanding of exactly how little I know about this lutherie game!


Sent from my iPhone using Tapatalk