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Finish fail - need to back-up a few steps

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Finish fail - need to back-up a few steps

Postby Steve Sawyer » Thu Oct 12, 2017 9:10 pm

I finished the neck on my Tele build. Turned out absolutely gorgeous. I didn't sand-through anywhere, my sanding schedule worked just fine, and the polished lacquer looks like a mirror. Two tiny spots (one about 1/8" diameter, just behind the nut, one a little 1/4" x 1/8 spot on an edge of the headstock) ended up with a few scratches, and I was thinking of going back and hitting them with the 1200/1500/2000 and re-polishing, but to be honest, they're one of those things that I'd have to point out to someone to be noticeable.

However - bad news on the body.

I ran into a couple of minor problems when doing the neck which I attributed to contamination while progressing through the grits. A couple of times, on examination before progressing to the next grit, I could see some deeper scratches, and had to work on it a bit more to get it ready to progress to the next grit. This was all wet-sanding - 400/600/800/1200/1500/2000 before polishing with Meguiar's 105. With the experience of the neck behind me, I got real fussy when I went to work on the body.

Same schedule of grits, but I soaked the sandpaper overnight, and while working I was real careful to change the water I was using to dip the sandpaper into while working, so I wouldn't get grit contamination from the previous step. I also started each grit with a couple of fresh dry shop-towels to wipe off the surface, again so as to avoid carrying any grit from one step to the next. I also examined every square millimeter of the surface to verify that I saw a completely uniform scratch pattern before progressing to the next finer grit.

I spotted some dimples after block-sanding with the 400 that I drop-filled, and waited a week for it to cure fully before scraping flush with a razor blade. No sand-throughs and was careful, careful, careful to get that uniform scratch pattern before moving to the next grit.

Everything went well, until I started polishing, and was presented with fine scratches all over the body. It looks horrible.

So - what the hell did I do wrong, and where do I go from here?

I block-sanded with the 400 and 600 just like I did with the neck and headstock, and progressed through the same grits that I did with the neck (800/1200/1500/2000) using just my fingers. As I mentioned above, I was more fastidious in my work on the body than I was on the neck. I'm wondering if maybe I didn't clean the sanding residue off of the surface sufficiently before doing my visual examination. Perhaps some of the sanding swarf filled the scratches, making it LOOK like it was ready to go to the next grit, when in fact it needed more work. Then again, the neck is natural maple, but the body is clear coat over a color, so that may have caused a failure in the visual check.

As to getting myself out of this mess (once I have some idea how I messed up), my thinking is that I could back up and repeat the entire sanding process, but given that I've done this once, I'm really asking for a sand-through if I try to repeat it (I had 8 three-shot coats on it before I started sanding), so if I'm going to back up, I might as well do it right. We're going to have some sunny, dry, reasonably warm weather in the next week, and the humidity should be good. I'll get a couple more cans of lacquer (I've been using Behlen Stringed Instrument Lacquer), and lay down three or four additional coats.

If this is indeed a good option, what should I use to clean the surface of the existing finish? I've had water with dish-soap, sandpaper and Meguar's 105 all over this thing, and don't want to compromise the new lacquer coats.

Any thoughts on my recovery plans, and suggestions as to how I screwed this up are appreciated.
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Re: Finish fail - need to back-up a few steps

Postby Barry Daniels » Thu Oct 12, 2017 9:34 pm

You started with too coarse of sandpaper. I start sanding after the final coat with nothing coarser than 800. It is very difficult to remove the deep scratches caused by coarser sandpaper like 400 grit. You can see if deep scratches are removed if you wipe the surface down using a damp microfiber cloth. Microfiber auto detailing cloths are the bees knees for guitar finishes.

I would recommend cleaning the surface with naphtha or mineral spirits and spray 2 or 3 coats of lacquer. Also, do not sand with your fingers. Use a soft block like a thick felt pad or a rubber eraser.
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Re: Finish fail - need to back-up a few steps

Postby Barry Daniels » Thu Oct 12, 2017 9:40 pm

One approach I have used successfully is to spray and sand until I get a perfectly flat surface and this sanding between coats can be done with 320 or 400 grit sandpaper. Now spray two good wet coats which are basically intended to fill in the scratches from sanding. Now you have a good flat surface ready for polishing, and you can probably start sanding with 1000 grit sandpaper. And this will only create very shallow scratches that can easily be removed by 1500 paper.

What are you using to polish the surface with the Meguiars 105?
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Re: Finish fail - need to back-up a few steps

Postby Aaron Helt » Thu Oct 12, 2017 10:18 pm

Yep, I start with 800 also. 3m dry. I go straight to buffer.
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Re: Finish fail - need to back-up a few steps

Postby Steve Sawyer » Fri Oct 13, 2017 12:49 pm

Barry - thanks. That sounds like that (starting with a too-coarse grit) was probably my mistake. The good news is I have a beautifully flat surface to work on at this point, so a couple of good wet coats should get me back on track. :roll:

I have this spiffy 3" random-orbit polisher from Griot's Garage that I was planning on using. However, I was a little afraid of it to be honest. I wasn't sure which of the foam pads that I got with it (it was a gift last Christmas) I should use, as I couldn't tell the difference as to their firmness (they all felt about the same to me) and don't know what the difference is as to the effect of the pads with a flat-face, vs. the ones with an egg-crate surface. I also don't know how much pressure to apply to get good cutting without burning through the finish. Also, to be honest, that foam doesn't seem like something I want to put on a guitar finish. I was thinking of figuring a way to affix a micro-fiber cloth to the pads which seems to to me to be a much better choice for polishing, but what do I know? Given that, I made a small (like 1 1/2" dia) polishing pad from some well-washed cotton (old t-shirts) similar to what you'd make for applying a french polish (damp paper shop towel in the middle, and wrapped tight with the cotton) and did the neck entirely by hand that way, so I continued to use that technique on the body.

Since I had nothing to lose, I experimented with some micro-mesh just to see how it compared to the polish, and started using micro fiber cloths to wipe off the surface. I realized that was probably a better approach than paper shop-towels, and was planning on using those on the next go-'round. Thanks for confirming that! :)

As to a backing pad, I'm using a very small block for block-sanding (about 1" x 3/4" x 3/4") and sanding in an overlapping circular pattern. Would you recommend sticking with a similar sized felt or rubber pad when working through the grits? I want to be able to sand right up to, but not over the edges to avoid sanding through. Using very small squares of sandpaper and working like this worked very well on the neck. Also, if I use a felt pad, am I correct that I should use a different pad for each grit so I don't inadvertently transfer an errant piece of coarser grit when switching to a finer grit?

As you can see below, the neck came out great. Now I just need to get the body to match!!

Finshed Tele Headstock_s.jpg


Finished Tele Neck Back_s.jpg


Finished_Tele_Neck_s.jpg
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Re: Finish fail - need to back-up a few steps

Postby Alan Carruth » Fri Oct 13, 2017 1:29 pm

I've been using Meguiar's 'Unigrit' sandpaper for several years now. It's a Japanese product, and they've worked out a way to reduce the variation in the grit size, so it's much less likely to leave scratches. I'm finishing with varnish, which is a bit softer than nitro, so you may need to be more aggressive. If the finish went on level I can usually start with #2000, although I often just go with #1500 to speed up the initial leveling. After the #2000 paper I go to rottenstone, and then on to Novus polish. You'd use some of the Meguiar's stuff that's made for lacquer.

One thing that helps is to get into the habit of sanding in a different direction when you switch grits. I sand along the grain with 1500, go in circles with 2000, and so on. It's easy to see when you've eliminated all the marks from the previous grit. The temptation is always to stop too soon while there are still scratches from the coarser grit. The fine abrasives won't take them out and they stand out against the otherwise nicely polished surface.
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Re: Finish fail - need to back-up a few steps

Postby Steve Sawyer » Fri Oct 13, 2017 1:55 pm

Alan Carruth wrote:One thing that helps is to get into the habit of sanding in a different direction when you switch grits. I sand along the grain with 1500, go in circles with 2000, and so on. It's easy to see when you've eliminated all the marks from the previous grit. The temptation is always to stop too soon while there are still scratches from the coarser grit. The fine abrasives won't take them out and they stand out against the otherwise nicely polished surface.


Thanks, Alan - that's a GREAT method!! As to my sandpaper, I use only P-graded abrasives (the ISO/FEPA Grit designation vs CAMI Grit designation), which as I understand it, offers the benefit you mention - the abrasive particles are much more consistent in size. However, this also points up a point of confusion. ISO/FEPA (P-grades) are roughly the same grade as CAMI papers of the same nominal grade but only in the lower grits. Starting at about P-400, they begin to diverge significantly, so that a CAMI 1000 is equivalent to ISO/FEPA 2000 grit (both 10.3 microns).

Thus it behooves me to ask - when Barry and Aaron recommend finish sanding at 800, are we talking P-800 (21.8 microns) or CAMI 800 (12.6 microns)? Is there any "standard" or "common practice" in lutherie in making assumptions as to the type of grading system is being used when referring to sandpaper grits?
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Re: Finish fail - need to back-up a few steps

Postby Barry Daniels » Fri Oct 13, 2017 3:10 pm

Sandpaper recommendations is a mixed bag so it is wise to know which system we are referring to. My 800 grit is the P system, 3M 216U Fre-Cut. I love the stuff. Post a comparison chart on your shop bulletin board so you know equivalent grits.

I would really like to encourage you to use the Griot's buffer. Just jump on in, it will be fine. It is hard to burn though with a system like this. Apply a bit of pressure to start then let off at the end. Quit when the whiteness of the polish dissipates. If your foam pads all feel the same they will probably work the same. I like the flat pads instead of the egg crate. Messing around with coverings for the pads is probably not a good idea. Start off with the stiffest pad and finish up with the softest. Or just try one and it may be all you need.
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Re: Finish fail - need to back-up a few steps

Postby Barry Daniels » Fri Oct 13, 2017 3:20 pm

The neck looks great, but like I've said before, they sort of polish themselves. Surfaces on the body will take 2 to 3 times the effort.

Definitely try out my eraser idea for a sanding pad. Best thing I have ever found. The Pink Pearl is the one.
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Re: Finish fail - need to back-up a few steps

Postby Steve Sawyer » Fri Oct 13, 2017 5:09 pm

Barry Daniels wrote:Post a comparison chart on your shop bulletin board so you know equivalent grits.


Yah - I have a chart I copied from Wikipedia on the inside of the cabinet with my finishing supplies - along with mixing ratios for various epoxies and shellac cuts.

I'll go ahead and give the buffer a try when I get back to that point. I'm sure that properly used, it can give me a better (or at least easier) result. I'll also give your eraser tip a try. I thought that I'd have more control with my fingertips, but I'll defer to your experience. You can't beat the price!!

Looks like Monday thru Thursday of next week I should have good "spraying weather" so I'll get the body all cleaned up and ready to get back at it...
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Re: Finish fail - need to back-up a few steps

Postby Barry Daniels » Fri Oct 13, 2017 6:43 pm

Figertips are too small to spread the pressure out so you will get deeper scratches that way.

Similar to your Griot buffer, I use a Chicago pneumatic polisher for final sanding and buffing using 3" diameter pads which is perfect for guitar work. I do final sanding with 2000 and 4000 grit Abralon foam backed pads which do a great job of removing previous sanding scratches. The buffing compound has an easy job getting to gloss from there.

https://www.amazon.com/Mirka-8A-203-3000-Abralon-Polishing-Buffing/dp/B00ATR7BW4
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Re: Finish fail - need to back-up a few steps

Postby Steve Sawyer » Fri Oct 13, 2017 6:50 pm

Thanks for that recommendation, Barry. Those would work great with my Griot buffer.
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Re: Finish fail - need to back-up a few steps

Postby Barry Daniels » Fri Oct 13, 2017 6:57 pm

Yeah they would. They are pretty special. One pad will do a whole guitar. They can be washed out and used again. And they can be used by hand. Just be aware that they have a 1/4" thick foam backing so there is no leveling effect. If you use it too much you will start to get a wavy surface, but at 2000 or 4000 grit that will take a while. You should also stay away from the edges of the guitar with these pads.

I also have a 1/2" thick transition pad that I can place under the Abralon pad which allows me to go right into the waist of a guitar.
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Re: Finish fail - need to back-up a few steps

Postby Steve Sawyer » Sat Oct 14, 2017 12:18 pm

Barry Daniels wrote:Now spray two good wet coats which are basically intended to fill in the scratches from sanding.


Just to make sure I don't go running off a cliff here - by "two good wet coats" are we talking the "coats" that I've come to understand from reading this forum? That is, a "coat" being three shoots, waiting 20-30 minutes between shoots, then letting the "coat" fully dry (a few hours) before doing the next set of three shoots?
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Re: Finish fail - need to back-up a few steps

Postby Barry Daniels » Sat Oct 14, 2017 12:40 pm

I have never heard of "shoot" terminology. When I said "coat" that means one spray session. Not three.

You want enough lacquer to fill the deeper sanding scratches, but not so much that you start to get unevenness. In my experience one coat is not quite enough to fill deep scratches left by 320 grit. However, keep in mind that I am using a traditional spray gun with thinned lacquer and your spray cans are another setup entirely. So take my recommendations and modify as necessary.
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Re: Finish fail - need to back-up a few steps

Postby Steve Sawyer » Sat Oct 14, 2017 9:36 pm

Barry Daniels wrote:I have never heard of "shoot" terminology.


I probably mis-referred to the method - spraying three times in quick succession, waiting for each application to dry to the touch between each spray, and referring to this as a single "coat". Didn't know how else to describe it.

But thanks for the clarification, Barry. Glad I asked. I'll see how it goes with these rattle-cans next week...
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Re: Finish fail - need to back-up a few steps

Postby Brent Tobin » Mon Oct 16, 2017 9:47 am

I use a Fuji HVLP unit and it does spray a heavier coat. (more material/less air)
So adjust as necessary
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Re: Finish fail - need to back-up a few steps

Postby Jeff Jewitt » Tue Oct 31, 2017 12:51 pm

For Aaron Helt ---

<<<Yep, I start with 800 also. 3m dry. I go straight to buffer.>>>

What compound do you use after the 800. Also is it P800?

Jeff
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Re: Finish fail - need to back-up a few steps

Postby Aaron Helt » Tue Oct 31, 2017 1:30 pm

Jeff, I'm not sure what grit the buff compound is. But, I can tell you that it is the dry, bar type stuff (Menzerna) and it is red. Then I follow with the medium (Beige colored) then fine (light beige).
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Re: Finish fail - need to back-up a few steps

Postby Jeff Jewitt » Tue Oct 31, 2017 1:44 pm

Its probably 113GZ (Menzerna's #). I ask because I've never tried going straight to buffer after 800.

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