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I'm currently finishing a mahogany neck with polyester and I'm having a problem that driving me nuts. I keep getting spots that look like sand-through although I don't think they are. They are more pronounced when viewed from an angle. In fact, some of them disappear when viewed straight on. Some of them I got rid of by sanding back and re-finishing only to have others show up in a different spot. I've been chasing these things for hours.

The thing is, I don't really understand what's going on. As I said, I don't think these are actual spots where I've sanded through. I thought perhaps they were from some kind of contamination such as residual glue but now I don't think so. I only seem to be having the problem on the mahogany. Is it something to do with mahogany?

If anyone can shed some light on what's going on and what I might do, it would be much appreciated.

Thanks,
Pat

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There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

Without seeing the issue, I would only be guessing.
Do the "spots" form irregular shaped rings? If so, the spots could be "witness lines". They can show up when you sand through one coat, into the coat below. They are prevalent in finishes that don't fully burn in from one coat to the last. Nitro doesn't normally have this problem, but polyurethane & polyester can, if the finish is sprayed over a cured coat.
If this is your "issue", you will probably have to sand well back & re-spray.

it could be big bubbles patches, or blisters, where the polyurethane came free of the wood, underneath, so the void takes on a lighter color. i saw this in a mexican strat neck once, looked like it had been dipped in molten plastic rather than sprayed....it bubbled in large patches like you describe. horrible stuff for instruments.....

They aren't witness lines - they aren't rings and are apparent prior to any sanding. The idea that it might be areas where the polyester has not adhered to the wood I find unlikely but interesting. I will think about that.

Thanks for the ideas. Any others? I'm getting set to start again today. I'll try to get some pictures.

Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

OK, I'm at it for the second day. Here are some pictures:

This first one is to show the spot I'm getting on the heel. I was getting a spot in the same location yesterday. This is after sanding back the whole neck and starting again. I didn't start seeing it again until about the 1000 grit sanding. I got frustrated and put a blob of finish on it, cured it and sanded again. I can assure you, that at this point the finish over the spot is thicker than the finish around it. I should also point out that I have managed to get rid of some spots by sanding and refinishing.



The next picture is another spot near the 12th fret position. Yesterday the spot was bigger so I have made some progress in this location but there's still a problem.



The third picture shows the heel spot after a bit of buffing. Buffed up, the spots diminish somewhat when viewed head on:



However, when viewed from an angle the spots jump out as seen in this last picture:




As I said in my original post, I really don't know what's going on here. Since I don't know what else to do, I'll be sanding it all back once again but I feel like I'm getting no-where. Have the pictures given anyone any ideas they can share with me?

Thanks,
Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

Pictures would help. You're using UV cured stuff right? My first thoughts are, are you letting it off gas long enough before curing? What kind of sealer? How long is the sealer curing? Is the UV curing heating the wood up to the point you're cooking resins out of it? (unlikely) Polyester, reguardless of UV or catalyzed is very sensitive to any contaminates on the surface being finished. I wash mine with solvent several times during the finishing process. If you are washing the surface with solvent you have to wait a while for all of the solvent evaporate.

I use Axon Polyester (not UV cured). I can apply the top coat as quick as 30 minutes after applying the sealer, but they caution to let the sealer cure longer if you're using a UV cured top coat. One of my reasons for using Axon is there's actually a chemical bond between the sealer and top coat, not just a mechanical bond. (and no, I don't fully understand all that chemical stuff LOL)

Again, I don't use the UV cured stuff (I used solarez on one guitar) Maybe someone who uses UV curing will have better suggestions.

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

Wow, Pat... That is so weird! I can see why this has stumped you!
The only thing I can think is there is some kind of contamination in the mahogany, preventing finish penetration.
I hope you find a fix... Pretty frustrating!
Have you considered sanding back & applying a dark stain? Not the ideal solution, but it might save the neck, if the stain will penetrate evenly.

Pat,

I don't really have an answer for you but I had the same thing happen to me with a mahogany neck on the uke I built. I sanded back and refinished several times. Even to the extreme of intentionally removing wood. It always came back. My experience was with the Stew Mac brushing varnish.

I ended up tinting some of the varnish to try and get it to match. I just looked at the uke and I can still see where it was.

Good luck,

John

Pat,
Did you hold or handle this neck with your bare hands just before finishing? After sanding back, did you wash these areas with naphtha or alcohol before re-applying finish? I am asking because I once did have some problems where my natural skin oil contaminated a neck in a few places just like this. Once I got my hand oils thoroughly washed off, I had no further problems. Of course, I am tossing this out as a "guess" answer here, but it's something to consider.

Patrick

Pat,

So far, I think you've got the best possible explanation. Two days so far and I'm not much further ahead. I sanded back and tried again but had all kinds of blotches on the front of the heel. Not sure what happened there. I sanded some more and it looks like the heel will be OK but I've had enough for today.

Tomorrow I will sand some more, put on some rubber gloves, clean it all up and try again.

Thanks for your thoughts.

Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

Looks very much like a sand-through to me. Some finishes/fillers like epoxy and polyester absorb some colour from the wood, so leave a slightly bleached effect if you re-coat directly over a sand-through, giving the exact look that you've photographed.

On rosewood, it is possible to re-stain the wood to match by picking up some colour from an off-cut on a solvent soaked rag and wiping it on the sand-through. (Takes some practice, like all staining, to get it invisible, though). You could try the same procedure, though is hard to get sufficient colour density on the rag on light coloured woods.

If you can't get that to work, its going to have to be a sand back to bare wood all over, taking off sufficient to get beyond the bleached stuff. Yes, it's a real PITA.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Polyester doesn't adhere to wood well. You need to spray the guitar first with the isolater, and then the polyester over that. The isolator sticks to the wood, and the polyester sticks to the isolator, but polyester won't adhere to wood directly very well.

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Old growth, shmold growth!

I don't know how I missed the pictures before making my first post, but it looks to me like you sanded through the sealer, before spraying the polyester. Sand back to wood, re seal, then apply the top coat.

_________________
I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

Well I finally got it. I put on some vinyl gloves, sanded down again and wiped it all with alcohol and re-applied the top coat. After that I was able to sand and buff it up without new spots appearing. That was a tough two and a half days!

To be honest, I'm still not sure I understand exactly what the problem was but I do know in future, I will be wearing gloves and wiping down with alcohol prior to doing the top coats. I still think it was something more than just sanding through and oil contamination off of my hands seems as reasonable explanation as any but I guess I'll find out on the next build.

Thanks to everyone that offered suggestions.

Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

I second theguitarwhisperer. We sprayed hundreds of mahogany guitar necks with poly and never had any problem like you described, but we never tried it without isolator!!

I have to revise my last message. It should say: "I thought I had it". In removing the masking tape I noticed a spot when the neck was on an angle that I didn't see when looking head on. It was right on the back of the neck at about the third fret location. I was so frustrated I was tempted to leave it but after about an hour I was back at it. Though I knew it was unlikely to work, I tried to see if I could fix the one spot and live with some witness lines. Also as an experiment, I re-applied the isolator/sealer where I sanded off. It all seemed to work for the spot I was trying to fix but naturally, in trying to minimize the witness lines, I sanded through in a area beside the area I was fixing. Just as well I suppose. I might have been tempted to leave some witness lines. So I sanded it all back again and as I type this a fresh coat of isolator/sealer is drying. That will be it for tonight though, I'm bushed.

Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

Pat,I admire your never say die attitude.I wish I had some advice that would help you.
James

_________________
James W Bolan
Nashville Tennessee

Hi again, Pat. I have no doubt you are hugely frustrated. However, if you've done a complete sand back and started over, I bet you'll be fine this time. I sure hope so. Seems to me this could have been any combination of things happening to you. Good luck with it this time, and let us see it again. I think a lot of forum members can benefit from your misfortune and your subsequent determination to solve the problem. I admire your determination, too.

Patrick

Thanks for the words expressing admiration for my fortitude however I'm starting to feel like the crazy guy that just keeps doing the same thing over and over and expecting a different result. I've been through the same cycle again today. I applied the topcoat, sanded back only to find some spot show up at the higher grit paper. Tried to individual spots again to no avail so I am once again in the same spot as I was last night. I've sanded back and re-applied the sealer. I think this time I might do two coats of sealer and a layer of pore filler just for fun.

I don't know if this is a clue but when sanding back, the trouble spots seem to go back to bare wood quickly whereas other areas take a ton of sanding. This suggests that the spot areas, if not sanded through, are at least thinner. Why there should be thin areas of finish, I have no idea. Does wood have areas that can suck in a finish. Also, by the way, is mahogany considered an oily wood? The sealer I have that came with the cureuv.com kit says that it is necessary only for oily woods.


Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

Pat, your description of what is happening suggests to me that you are only sanding back in spots. I don't think that will work for you. In my previous post, I used the words "complete sand back." By that, I meant sanding off all traces of finish and sealer coat all over the neck. You could probably save the face of the headstock and maybe the sides, too. But if it feels like you're not getting down to bare wood on the rest of it, you probably aren't. Take everything off the back of the neck. Everything. Keep your work clean (a glove on the hand that actually manipulates the wood). Wash it all thoroughly with alcohol or naphtha. Re-apply your sealer overall (except maybe those areas of the headstock that you didn't sand completely back. Then go for your finish. If you have the patience to do that, I think you will be okay. The devil is in the details when solving a problem like this, and since you don't know what is specifically causing the problem, you've got to address all the potential causes.
Reading you latest post, I am now inclined to think you're just trying to fix spot sand-throughs. Maybe some contamination, too, but I think mostly areas where you're trying to match bare wood touch ups to areas that still have sealer and finish. Doing that is not impossible for a finishing expert, but it's not easy, either. Go one more time for a full sand-back. I think, six months from now, you'll be very glad that you did. And you'll have a heckuva story to tell you grand son one day!

Patrick

Finally, finally I got it for real this time.

Patrick, I took your last message to heart and spent two and a half hours sanding the neck back to bare wood. In cleaning it up with alcohol, I noticed a few spots that didn't seem to be going dark so I sanded them some more until the whole neck looked uniform when wet with alcohol. Then I put on a coat of insulator and the colour stayed uniform. After that dried, I applied the top coat but that turned up some very blotchy areas on the heel and in the volute (i.e. on end-grain wood). Here is what the heel looked like when I first applied the top coat:



If anyone can tell me what's going on there, I'd appreciate it. Anyway, I wiped it off without curing it and cleaned it with alcohol. I had seen this before in my earlier tribulations and knew that some sanding would help. So I worked on the blotchy areas by sanding them very, very lightly with 320 grit paper. I was not sanding through the insulator. After each bit of sanding, I tested for colour by just wiping on some of the top coat. I'm not even sure what the sanding was doing, but it got rid of the blotchiness and it wasn't too long before the whole neck was a uniform colour. Then I applied the top coat to the whole neck again and cured it. I sanded and buffed it up and now it looks fine.

This has been a tough three days. I've never come so close to launching part of a guitar through a window.

Thanks again to those that provided suggestions.

Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

Congrats Pat.Give yourself a day off .Drinks some beer ,have some fun.

_________________
James W Bolan
Nashville Tennessee

Yo, Pat,
That last photo looks pretty scary to me! Wow! If I understand your post correctly, you have now overcome those problems on the heel--is that correct? If so, my hat is off to you. Pat, I was operating on a series of hunches when I posted earlier. I told you that. I don't know whether my comments actually helped you, but I guess they encouraged you to make one more pass at the thing. I think you solved this problem all on your own, and I am thankful for that and happy for you. But, man....what could have caused that weird problem?

This problem is bound to come up for some other builder sooner or later. I wish we could figure out definitively what caused the problem--for the sake of everyone here.

Regardless of that, congratulations on your perseverance and a successful outcome! You 'da man!

Okay, my friend, let's see this baby when she's all buffed out and strung up. Okay?

Patrick

Thanks again everyone. Guess I should close the loop with a few pictures of the finished neck:










The back of the head is Brazilian Rosewood which came from back scrap. The front of the head is ebony with Japanese Maple Leaves from Andy DePaule.

Cheers,
Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

Beautiful! A testament to your perseverance!
Patrick