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David - Machinist in wood
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Discussion Starter · #1 ·
This project represents several firsts for me -

1) First acoustic guitar from scratch. I've replaced tops, backs, bridges, saddles, nuts, done inlay, repairs, etc. but this is the first one from scratch - resawed the wood, bent the sides, etc.

2) First time to do a French polish from the start and not just a repair.

3) First time I've made this many mistakes in a project and kept going, trying to figure out how to successfully fix what I've done and trusting it's still going to work out ok.

So here's the sanding tip I learned a long time ago and I have no idea if it's something I read, something I figured out, or even if it's common knowledge - It takes twice as long sanding with the next grit as you spent sanding the previous grit.

What do I mean by that? If you're sanding a finish, or even bare wood, with say 220 grit and you move to 320 grit, then if you sanded for 5 minutes with 220 then it's going to take 10 minutes of sanding with 320 to remove all of the 220 grit scratches.

Right now I'm wet sanding the guitar that has a very thin film of shellac and when I wet sand with 320 it takes no more than a minute to do the back twice. When I switch to 400 I sand for about 2 minutes although I don't time it. Basically I sand the back twice, wipe the slurry off, blow it dry to see if I have even coverage of sanding, and then switch to 400 and do the same thing. Only now with 400 I do the back about 4 times. When I switch to 500 I'll do it 8-10 times. When I get to 600 I'll be doing it at least 15 times. By the time I get to the 1200/1500/2000 I'll probably keep going until it looks right and then switch to Micromesh.

I haven't made it past 500 yet because I keep seeing where I'm getting too close to burning through to the Mahogany so I've had to stop and shellac again several times. So when I get to the finer grits it's necessary to judge how much finish is left so I don't go through on the polishing later.

Anyway, it's a sanding tip I've passed along to lots of folks so while I'm waiting on shellac to dry it seemed like a good time to post this (only takes a few minutes to dry before I can sand again).

Wet sanding
Guitar Wood stain Musical instrument Varnish Table


String instrument String instrument Musical instrument Violin family Bowed string instrument


Fresh shellac
Wood stain Table Wood Paint Furniture


David
 

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Hmmm...
In effect you're saying that each time you sand you're taking everything down to the level of the bottom of the deepest scratch.
My attitude, reached after learning from drywall finishers, is that you remove the high spots...and that's a bit subjective...then fill in all the low lying landscape with the next coat. All scratches disappear until you start sanding again; this time with a finer grit.
A different approach, with the basic difference being that the depth of finish is building up in thickness, not being reduced to very close to what it was on the previous coat.
I'd love to hear from any of the car restoration guys here; what's your process with multiple lacquer coats?
 

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As a side note, in the Drywallfinishing final practical exam up here, the students aren't allowed to touch the project with sandpaper* until the final coat of mud is dry. ie the approach that I described above. Each successive coat buries the low lying faults.

* not suggesting that's a perfect analogy; just a different perspective.
 

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I haven't made it past 500 yet because I keep seeing where I'm getting too close to burning through to the Mahogany so I've had to stop and shellac again several times. So when I get to the finer grits it's necessary to judge how much finish is left so I don't go through on the polishing later.



David
....and thats the key to success David.... knowing exactly where you are at within the process!!! Messing up at this point can be fixed, but not without alot of work. Pay very close attention to almost every single detail, every scratch, every low spot, every high spot, the edges, the curves, the pressure on the sanding mediums and for me, most importantly every "reflection". In the picture I added, I'd give the job an 8 outta 10. The reflection is the tell of the tape. This particular job was finished off with Minwax rattle can lacquer, a wet sanding schedule very much like what your doing and then I used Griot's Garage car polishes. Prior to using the Griots product, I used McGuiars car polishes.

Beautiful job thus far David....
 

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David - Machinist in wood
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Discussion Starter · #5 ·
Kind of the same thing, Dan. I use the 320 to level the surface, even though it is very flat and level now but I make sure there aren't any shiny spots I've missed. The 400 makes the scratches even finer but if you don't do it enough then there are deeper scratches that the 500 or 600 has trouble removing. If you use the next finer grit to remove the 'coarser' grit scratches it just takes longer until they're all gone. In this case with a high gloss sheen the scratches from the 320 would show if I didn't remove them with the 400. The 400 scratches would show if I didn't remove them with the 500, etc.

So what I've found is that it takes roughly twice as long with the next finer paper to remove the scratches from the coarser paper. Whether you're sanding 100 grit on bare wood and going to 120 grit or sanding with 180 and going to 220 and on to 320. People have asked me how long on each grit and the general guideline I've given is what I've stated here - about twice as long on the next grit as you did on the previous grit.

David
 

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David - Machinist in wood
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Discussion Starter · #6 ·
Thanks, Bill! And that's a beautiful chess board - wow! I'll be using Meguiars once I finish with the Micromesh papers.

Another good tip on sanding is get the edges right and the middle sort of takes care of itself. All too often the edges are the indicator of a good finish job. It's easy to work on the easy middle part for finer sanding but people are afraid of burning through or rounding edges and a lot of finishing jobs I've seen show that people stayed away from the edges for that reason.

David
 

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@DaninVan

I'll just give you my own perspective and try to answer your query... You're pretty much spot on with the "scratch" concept. You take out the bigger scratches with smaller scratches. With a mirror finish such as David's, you're taking that scratch pattern down to the point of not being able to notice the scrach's at all. And you keep on going from there. You're not only working the finish, you're also working the wood as well. Not only does the laquer have its high/low spots, but the wood itself has em. Once you get to the point that you are satisfied with the wood you move onto the finish. But even the best prepped pieces of wood will have its own high/low spots when you are attempting to achieve this level of a finish. Now you can either work those first few coats of finsih back down to the high spots in the wood and build up the low spots in the lacquer or avoid going back down to bare wood and just work on building up the low spots in the lacquer. Personally I find larger flat panels MUCH easier to deal with than those such as David is working with here. Curves, bends and bows all add to the difficulty of creating a surface that just flows smoothly and evenly. This is where I've come to rely on the reflection. The crisper and the sharper the reflection the closer you are to where you wanna be. Hope that kinda sorta maybe helped..
 
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David...

DO NOT give the car polish a try until you have tried it and experimented with it. I'm sure I don't have to tell ya how I know this *L*
 

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I've never heard of using such fine grits. I would use a card scraper to get a near perfect, high gloss surface. All that sanding has got to raise the grain, and you're going to lose a LOT of wood in the process. A card scraper requires a bit of practice, but makes a beautiful surface.
 

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David - Machinist in wood
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Discussion Starter · #11 ·
David...

DO NOT give the car polish a try until you have tried it and experimented with it. I'm sure I don't have to tell ya how I know this *L*
Thanks, Bill, but I've been using this Meguiars on guitars for over 30 years. Works great!

I've never heard of using such fine grits. I would use a card scraper to get a near perfect, high gloss surface. All that sanding has got to raise the grain, and you're going to lose a LOT of wood in the process. A card scraper requires a bit of practice, but makes a beautiful surface.
The sanding isn't raising the grain, Tom, because it's all under the shellac. I did use a card scraper when the back was flat and before the braces were glued on. The Micromesh I have goes down to 12,000. I may not go that far but I have the option. At that point I can polish by hand without having to use a buffing wheel. Matter of fact, if I just go to 6,000 I can probably do the polishing by hand.

David... are you block sanding or just using your hands?
Block sanded the wood before the finish and using the same block now. No surprises that way. The only places not getting block sanded are on the neck in the small detail areas.


The Mahogany was sanded to 320 and then I used a card scraper. The shellac went on very smoothly but it still needs to be leveled if you want the pores filled, which I do. To fill the pores you can either use a pore filler - many different kinds available - or use the finish to fill the pores. I chose the latter. By applying the shellac and sanding it back down, then wiping the slurry across the grain, the pores were filled in no time. Each subsequent coat of shellac melts into the previous and as you keep sanding and taking the finish back down to a base level you finally reach the point where you can go to the finer and finer grits.

To add to what I replied to Dan @DaninVan earlier on the drywall and the low spots, in retrospect it is very different. As Bill said on the wood having high/low spots, that's something I took care of in the bare wood stage. The back on this guitar is only 0.100" thick so you can't just go 'sanding crazy' once it's together. If it was the 3/4" top to a table I'd have plenty of leeway on getting the top surface flat but on this curved surface it had to be right before I glued the first brace on (the back has a domed 15' radius). Obviously there were some very minor areas that sanding and scraping took care of but before the first coat of shellac went on the back was flat (except for the dome radius...).

So where the difference comes in is that with drywall once you get the wall flat you're typically going to texture it and then paint it. All of that serves to hide any sanding scratches and minor irregularities. With the guitar and a high gloss, very thin, transparent finish that luxury of having a cover-up opaque finish is gone. Every irregularity will show. So all of the scratches have to be so small that they can't be seen.

Same with painting cars - once you get the panels flat and sanded to as fine as you want to go the car gets sprayed with an opaque paint that is usually not sanded before the clear coat goes on. And unless you're doing a show car the clear coats of today don't get sanded and buffed out to a high gloss. The clear coat just dries to a high enough sheen that further buffing and leveling isn't required. It may have a slight bit of orange peel but that's also acceptable in most car finishes, to a degree.

The final way that none of this is the same is the film thickness. Guitars are built to sound good, not just look good. The thicker the finish the poorer the guitar sounds. So I don't have the luxury of building coats to the point I'm so far away from the wood that sanding through is no longer a possibility. Nope, the finish is somewhere around 2 mils to no more than 3 mils thick and I don't want it to go any thicker.

Nitrocellulose lacquer is what I've used on guitars before and it's much easier. There's a lot of debate as to which sounds better but that's another topic. I'm using a French polish technique with the shellac more because I wanted to see if I could get the finish like I want. I know I can get it in lacquer.

Either way it's a fun build!
David
 

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Thanks for the detailed explanation. Now I understand. I guess a thick coating will dampen the vibration and suppress the resonance. Very cool project, hope you'll post a picture or two of the final result.
 

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"The final way that none of this is the same is the film thickness. Guitars are built to sound good, not just look good. The thicker the finish the poorer the guitar sounds. So I don't have the luxury of building coats to the point I'm so far away from the wood that sanding through is no longer a possibility. Nope, the finish is somewhere around 2 mils to no more than 3 mils thick and I don't want it to go any thicker. "
-David

Well why didn't you say so? ;)
Seriously, now it makes sense. I had no idea that the finish thickness had that kind of tonal effect. Learn something new here everyday.
 

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Thanks, Bill, but I've been using this Meguiars on guitars for over 30 years. Works great!

David
Well dang... I'm preachin' to the choir :) Do you use a random orbit polisher at anytime during your finish schedule? With the right bonnet and
a lil practice I've managed to cut out alot of time in the process...
 

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Bill; that chess table finish is spectacular...well actually, the whole table is!
Well thank you sir!!! much appreciated.

yes, the table turned out pretty darn nice, if I do say so myself. Built it for a friend and his son, so they can spend some
quality time together just BS'n and playing a lil chess together. Mission accomplished :)
 

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David - Machinist in wood
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Discussion Starter · #20 ·
Well dang... I'm preachin' to the choir :) Do you use a random orbit polisher at anytime during your finish schedule? With the right bonnet and
a lil practice I've managed to cut out alot of time in the process...
The last time I polished a guitar I used a lambs wool pad in my drill press. I used to have a right angle polisher that I used on dining tables, conference tables, etc. but I don't have that now.

If I do the French polish correctly it won't need sanding or polishing. So far I've had to sand it back a few times. We'll see...

David
 
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