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Avoiding and dealing with sanding through
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Author:  joshnothing [ Mon Feb 02, 2015 8:57 pm ]
Post subject:  Avoiding and dealing with sanding through
Hi everyone,

I was wondering if I could ask for a bit of help when it comes to finishing. I always find this part of the process problematic.

I'm in the middle of a finishing process for a padauk jumbo w/ cutaway. Some of you were kind enough to help with me with a question about the fretboard extension a week or two ago.

The guitar is porefilled with zpoxy and finished with the U-Beaut Hard Shellac + Acetone method described in the Gore/Gilet book. It is certainly a departure from the book to fill with epoxy for this finish, but I haven't had any problems with adhesion. This is a totally new and experimental finishing method for me - all my previous builds have been Tru-Oil/Liberon Finishing Oil, a method I've practiced with enough to get good results from. I was motivated to try something else because I didn't feel it resulted in a durable enough finish. Wanted to try for something glossier too.

The finish coats were built, leveled and built again as described in the book. It was left to cure for 10 days as suggested and then sanded flat with Micromesh and Buflex as suggested in the book. It was left for an additional week as life got in the way, at which point there'd been some very slight sinkbank into the pores of the back and grainlines of the top. It was flattened again, this time with wet-and-dry paper from 800 to 2000 (My micromesh was worn out by this point).

Even unbuffed, I love the look and I love how hard this shellac seems to cure.

However, my inexperience has risen to the fore and just as I completed the final sanding (2000 grit), I managed to sand through in two places - one 1/8 inch long area on the purfling of the top, and one round patch the size of a dime inside the curve of the cutaway. The rest of the guitar looks perfectly flat and ready for buffing. Whoops!

I don't doubt for a second that this is due to operator error - I'm thinking I either levelled poorly before finishing, put insufficient finish coats on, or used poor sanding technique when levelling. Very possibly it was all three.

So I'll keep working on my technique for future builds. My question is how to best deal with these sanded through areas?

Should I refinish the entire instrument, or at least the entire 'panel' (eg. soundboard, sides)?

Can I get away with spot refinishing these areas (eg a patch a few inches square around the blemish)?

Currently trying to stay zen about the whole process and stick with it to 'learn' this shellac finish method, despite the temptation to use the nice smooth shellac surface as a base for few coats of Tru-Oil and call it good! :D

Any advice appreciated as always!
Author:  meddlingfool [ Mon Feb 02, 2015 9:47 pm ]
Post subject:  Re: Avoiding and dealing with sanding through
I'm not certain, but shouldn't shellac burn seamlessly into itself? Therefore making invisible spot repair possible?
Author:  joshnothing [ Mon Feb 02, 2015 10:58 pm ]
Post subject:  Re: Avoiding and dealing with sanding through
meddlingfool wrote:
I'm not certain, but shouldn't shellac burn seamlessly into itself? Therefore making invisible spot repair possible?


I hope so but I have some concerns as this hard shellac product claims to 'cross-link' during cure and become somewhat alcohol resistant. But this is my first attempt to use this product, so I'm not sure how soon that happens or whether it truly stops subsequent coats burning in.
Author:  Trevor Gore [ Tue Feb 03, 2015 6:48 pm ]
Post subject:  Re: Avoiding and dealing with sanding through
Shellac, done right, is a very thin coating, so you don't get many chances to level it before going through it.

It seems to me that you would be best off applying more shellac to the whole guitar. If you try to do a spot finish then blend it in, the chances are that at this stage you will sand or buff through somewhere else.

You don't mention how many sessions it took to get to where you were, but I'd suggest that you need at least half as many again over the whole guitar. If you don't feel comfortable at that stage, do the other half of the sessions.
Author:  joshnothing [ Tue Feb 03, 2015 11:52 pm ]
Post subject:  Re: Avoiding and dealing with sanding through
Trevor Gore wrote:
Shellac, done right, is a very thin coating, so you don't get many chances to level it before going through it.

It seems to me that you would be best off applying more shellac to the whole guitar. If you try to do a spot finish then blend it in, the chances are that at this stage you will sand or buff through somewhere else.

You don't mention how many sessions it took to get to where you were, but I'd suggest that you need at least half as many again over the whole guitar. If you don't feel comfortable at that stage, do the other half of the sessions.


Thanks for the response Trevor. By sessions, do you mean coats? Or sessions of applying coats? Geeze, that sounds confusing now that I type it.

I did 48 coats all up - 24 coats, then flattened with micromesh as described in the book, and then another 24.

To clarify, I was applying coats of shellac using a wet application fad. I would follow a process like this:

* Wipe three coats on the back, 30 seconds or so between coats, fan blowing on guitar to speed drying
* Leave to harden while doing three coats on another area like the sides
* Leave to harden while doing three coats on another area like the neck
* And so on, working around the instrument continually.
* Every time I hit about 12 or so coats on the whole instrument, I gave it 24 hours to rest before continuing

I've gone through around 300ml of base dilution Ubeaut hard shellac, if that helps give an indication of how much finish I put on. That 300ml volume is just the shellac itself - it does not include the acetone added to make the azeotropic mixture.

Thanks again for the advice and time. I'll break out the U-Beaut and do another 24 coats and re-flatten.
Author:  Trevor Gore [ Wed Feb 04, 2015 2:32 am ]
Post subject:  Re: Avoiding and dealing with sanding through
300ml sounds like a lot of shellac. It's not something I've actually measured, but my guess is that I'm probably using less than half that, but I can put it on flat and I rub very little off when leveling.

I never count "coats" either! A session is a period of putting shellac on until it's had enough for that session (helpful eh? :roll: ). It's one of those experience things and depends a lot on ambient conditions. It's had enough when it's too soft to go around again, but you're better off stopping a bit before that. That's as best as I can describe it. It's very much a feel thing. I'm usually getting close to finishing off after 3-4 sessions per surface, which maybe ~10 or so of your coats per session. I apply pretty dry.

So if you do ~20 of your coats and don't hit it too hard with the abrasives, I think you should be fine. If you find yourself rubbing through, it's likely because the wood wasn't flattened enough before you started and you end up thinning out the high spots too much. There's a lot more skill to sanding and creating flat surfaces than most people acknowledge. If you're suffering from the surfaces not being flat enough initially, use a softer sanding block (e.g. cork with ~ 10mm of foam rubber between the cork and the back of the abrasive). That should ride the lumps a little easier with less risk of going through.
Author:  joshnothing [ Wed Feb 04, 2015 3:23 am ]
Post subject:  Re: Avoiding and dealing with sanding through
Thanks Trevor, that really helps. I'm going to stick with this process for my next several instruments in an attempt to get competent with it. At the moment I think I am definitely falling down in all three phases - prep, application and flattening. No cure for it except to keep trying!


Sent from my iPhone using Tapatalk
Author:  Grunt [ Wed Feb 04, 2015 1:22 pm ]
Post subject:  Re: Avoiding and dealing with sanding through
As a side note pay very close attention to the grit conversion between micro mesh, euro, and american grits. You may be leveling with a much coarser grit than you think after switching to paper after using the micro mesh. MM includes a handy conversion chart with their products. This was a bit ad a problem for me as I was mixing abrasives on my last finish. Sanded through mine too.
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