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 Post subject: Uneven purfling width
PostPosted: Mon May 18, 2020 9:53 pm 
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Hey everyone, I have some questions about purfling.
I bought some .20 x .20 black/white fiber purfling from LMI and glued it in with black ebony wood binding at the same time using LMI’s yellow glue. I used StewMac’s brown binding tape and did use considerable pressure when taping the binding/purfling down.
The results I got were a little disappointing. The white portion of the purfling showed up as varying thicknesses in different places along the binding.
The questions I have are:
1. Is it possible to use too much pressure with the brown binding tape and a glue that is water soluble?
2. Would cyanoacrylate glue be a better method, taping everything in place first and then wicking it in?
3. Would gluing the binding in first with a spacer for the purfling be a better choice? Then glue in the purfling.
4. Would plastic purfling create a better result?

I can go along the purfling with an Exacto knife and clean the white dimensions up pretty good, but it has been time consuming and seems like unnecessary work that I have created.
Really appreciate any help.
Bill


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PostPosted: Mon May 18, 2020 9:55 pm 
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Have you scraped or sanded the purfling and binding down fully flush with the top yet? In my experience, when the purfling has looked like that, it still needed a bit more scraping and then it evened right up.

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PostPosted: Mon May 18, 2020 9:58 pm 
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J De Rocher wrote:
Have you scraped or sanded the purfling and binding down fully flush with the top yet? In my experience, when the purfling has looked like that, it still needed a bit more scraping and then it evened right up.


+1


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PostPosted: Tue May 19, 2020 10:14 am 
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Do not use an Exacto knife for leveling binding/purfling! Get a real cabinet scraper. They are absolutely necessary for this work. The Carruth scraper from Stew-Mac is a step up from normal card scrapers.


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PostPosted: Tue May 19, 2020 11:12 am 
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+1 to both Jay and Barry.


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PostPosted: Tue May 19, 2020 1:39 pm 
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Agreed on all of the above. Just a word about very flexible blades used as scraping tools: It is rare for me to have success when I try to do that. I guess there are some light jobs for which the flexible blade of an exacto knife, or a razor blade, will do the trick. But most of the time, the more rigid the scraper, the better. In fact, there are some jobs where I need to use a chisel as a scraper. Invest in some card scrapers and learn how to get a decent edge on them.


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PostPosted: Tue May 19, 2020 5:48 pm 
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I do have and use a Carruth scraper for the binding. I was talking about using the Exacto knife to go parallel the purfling to adjust the width.

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PostPosted: Tue May 19, 2020 6:10 pm 
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Assuming that your purfling is behaving like all the black/white type purflings I've used with binding and in rosettes, going at it with an xacto knife is not the thing to do. The part of the purfling that is proud of the top or back after gluing it on with water-based glue has a tendency to mushroom a bit resulting in that sort of uneven appearance. Scraping it makes it mushroom out even more because you are compressing it, until you have scraped it all away. That appearance doesn't go away until you have absolutely scraped it level with the top or back and have exposed the edge of the purfling that is within the joint. Even if you think that you have leveled it, all it takes is a couple thousandths of purfling remaining proud to give that uneven appearance.

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PostPosted: Tue May 19, 2020 6:27 pm 
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What Jay said. Keep scraping and the wide purfling will suddenly disappear and you will have an even line. I guarantee it.


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PostPosted: Wed May 20, 2020 8:01 am 
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In my experience, wicking in CA glue is much easier than using water based glues for attaching purfling strips to bindings. I use LMI kerfed lining clamps to hold the purflings in place and then lightly wick in the glue. Remove the clamps and wick more glue where the clamps were. The purflings won't swell and you can see what you are doing clearly.

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PostPosted: Wed May 20, 2020 8:10 am 
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Let me be a bit more obtuse. There is actually an important lesson in this topic. When scraping purfling it usually looks really uneven at the start. The purfling is proud of the surface so it is unsupported by the top and or binding. As you scrape, the purfling gets mushed down and around making it very ragged and uneven. There is a dramatic point that you reach, when you are level with the top or back, and the purfling IS supported by adjacent surfaces and the lines become very even. The first time you see this it is almost a revelation. The second time you sort of realize that this is a sign that the purfling is flush and you can stop scraping. I am naming this a Work Stoppage Indicator (WSI).


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PostPosted: Wed May 20, 2020 9:41 am 
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Well put Barry! I think we all have felt that sinking feeling of a bad purfling job only to be relived by the sudden improvement made when scraping. After a while, you just get used to it and forget that not everyone knows it. Explaining exactly what is going on is helpful.

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PostPosted: Wed May 20, 2020 3:35 pm 
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I'll take a stab at answers for Bill's questions:

The questions I have are:

1. Is it possible to use too much pressure with the brown binding tape and a glue that is water soluble?
I use brown binding tape and water-based glue too and the answer is no. The amount of pressure you can apply with the binding tape is self-limiting by the tape's breaking point. I apply the tape as tight as I can possibly get it just shy of it breaking and have never had any problems.

2. Would cyanoacrylate glue be a better method, taping everything in place first and then wicking it in?
IMO, water-based glue and CA glue methods are just different methods. One's not necessarily better than the other, each has it's own pros and cons. There are a fair number of builders who use the CA method. I use CA to install just the side purfling. I use Titebond for the binding and top and back purfling and, since that method works so well for me, I've never had any incentive to try the CA method for that.

3. Would gluing the binding in first with a spacer for the purfling be a better choice? Then glue in the purfling.
I have done that to install turquoise purfling, but that was because it required fitting lots of separate turquoise pieces which would not be possible to do at the same time as installing the binding. I think that doing that approach with standard black/white veneer purfling would unnecessarily complicate the process and increase the chances of ending up with gaps between the purfling and the binding or the top or back.

4. Would plastic purfling create a better result?
Never tried it, but I doubt it.

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PostPosted: Wed May 20, 2020 4:07 pm 
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To me the plastic purfs look “too perfect” with whites that are just “too white” next to most natural woods on an acoustic guitar. But they look great on an electric where there’s solid paint colours or vibrant dyes/stains involved.


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PostPosted: Wed May 20, 2020 5:48 pm 
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Thanks Jay and Josh. I appreciate the suggestions and opinions much.

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PostPosted: Thu May 21, 2020 8:35 am 
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So, did you scrape it down yet?


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PostPosted: Thu May 21, 2020 9:50 pm 
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Hey Barry. Yes I did scrape it down (actually a little too much) and it did not change anything. First time I have ever run into this problem.

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PostPosted: Thu May 21, 2020 10:30 pm 
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That's weird. Don't know what to tell you.


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PostPosted: Fri May 22, 2020 9:01 pm 
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I'm now thinking that the purfling channel was a little bit too wide and the purfling may have tilted in places. Is that a reasonable guess?

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PostPosted: Fri May 22, 2020 9:35 pm 
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I would expect that if the only thing going on was that the purfling channel was too wide in places, you would just be seeing gaps at those places.

It seems to me that for the purfling to actually tilt, the channel would have to be too wide by quite a bit. Also, the channel would have to have been pretty shallow so that the height of the purfling exposed above the top or back was a lot when you taped it up and the tape pressure could actually push the purfling over. How deep was the purfling channel?

Can you tell us what the dimensions were that you used for the purfling channel and the binding channel and how you cut them?

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PostPosted: Fri May 22, 2020 10:41 pm 
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I cut a .050" binding channel first and then a .040" purfling channel next. For the binding I used a StewMac .050" bearing and for the purfling I used a .090" bearing from StewMac. I know SM gives an extra .010" on their bearings to allow for glue, but that shouldn't be too much. I'm wondering now if I may have gotten my SM and LMI bearings mixed up. Even then I wouldn't think that would matter.
The purfling channel depth was matched up just a very tiny bit shy of the purfling height.

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PostPosted: Sat May 23, 2020 12:44 pm 
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Those bearings in that order make sense. If anything, the 0.040" purfling channel would have given a snug fit for the 0.020/0.020 purfling when you add in the glue. With that channel width and having cut the channel depth so that it was just shy of the purfling height, I don't see how the purlfing could have tilted. And even if you mixed up those two bearings, at worst, you might get gaps in some places.

Did the uneven look show up everywhere or just in some places? Did the binding end up fitting tight all the way around?

When you said that you scraped the binding/purfling down a little too much, what did you mean?

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PostPosted: Sat May 23, 2020 6:35 pm 
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The uneven look just showed up in some places, but it was quite a few places and maybe a fourth of the total purfling. As far as scraping too much, well you could actually tell that the height of the Ebony binding is less in that area than the rest of the side. One more thought occurred to me. There may be a little more play in my router tower mechanism than I’ve had before.

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PostPosted: Sun May 24, 2020 7:13 am 
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I still think it needs more scraping.


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PostPosted: Sun May 24, 2020 8:14 am 
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Bill,

Your picture shows the white purfling line smearing over the binding and you state that the binding is "less than the rest of the side". This tells me that the top of your binding is lower than the plate. If this is the case then you will have to continue scraping to bring the plate and purfling down to the same level as the binding. You will not get a clean purfling line unless all three elements, the plate the purfling and the binding, are level. The other fix is to rout it all off and start over.

Steve


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