sanding

[Dan of SC]

I am a hobby builder and working on my 10th instrument. I have finished some with nitro and a couple with EnduroVar. With either finish I can get a good buildup, let cure 2-3 weeks, then level and sand up to 2000 grit, then buff with medium and fine compounds and get a very slick mirror finish, but if I hold it to a good light and look at a sharp angle, I see what looks like thousands of tiny hairline scratches. Before finishing I sand up to 220 grit, always with the grain and with some type of sanding block when I can. I usually sand between coats with 320-400 grit. I'm almost ready to start spraying this latest instrument and I sure could use some help here. Anyone got any idea how I can figure this out? Thanks, Danny Gray

[charles Tauber]

First, you need to identify whether the scratches are in the wood or in the finish. Look carefully under bright light. You should be able to see at what level the scratches exist, under the finish, or on the surface of the finish. A different remedy depending on what is scratched.

[Howard Klepper]

The kind of scratches you describe are likely created when you sand your topcoats. What grit do you begin with, and what comes between that and 2000? What brand and type of sandpaper?

If it's any comfort, eliminating those fine scratches is difficult. It is mainly about using good abrasives, keeping the work very clean as you sand, changing the paper frequently, and sanding very thoroughly with each grit. But some products work better than others.

[LouieAtienza]

When I put finish on, after my sealer coat, I never use anything under 800 grit, if I can avoid it. I do use a catalyzed urethane where I apply one sealer coat, level this to the wood after 24 hours, then apply the finish, let sit 15 minutes, and apply a flash coat. I can start "polishing" after a day, but I usually wait two days.

I also prefer wet-sanding, as the water (and a couple tiny drops of Palmolive) will float away particles that can scratch. I clean my water pail and use fresh water for each progressive grit, 1000, 1200, 1500. I also pre-soak my sandpaper to make the backing more pliable. I prefer not to use rubbing compounds, instead working with 3000 and 5000 Trizact pad. My preference is to stop with 5000 as I like a satin finish, but after 5000 I can go direct to a glaze compound and achieve a very high gloss with no visible scratching.

Whichever finish you use, you should ensure it is rock-hard, and should powder up with no build-up when sanding dry.

[Bruce Sexauer]

I sand wood surfaces to 220 except endgrain and some other sensitive spots, which I take to 320. Between coats I use 320 except the penultimate coat, upon which I use nothing coarser than 1000. On the final coat I use nothing coarser than 1500 for the last 100 guitars or more, and I have switched very recently to nothing coarser than 2000 grit. The reason for this fanaticism is exactly the issue you describe, Dan.

[Dan of SC]

I want to thank all of you guys for these responses. I really think I have learned from reading them. I am fortunate to have access to so much experience and knowledge! Danny

[mirwa]

Thousands of hairline scratches sounds like an issue predominantly in the final finish.

A mirror free finish to the human eye, still has tens of thousands of scratches, they are just so small and fine, our eye cannot differentiate between the highs and lows of the scratch, so to us it appears to be scratch free.


My process

http://www.mirwa.com.au/DIY_Paint.html

Steve

[redir]

Finishing has been a constant struggle for me since I started building. I'm on my 52nd guitar now and still run into these problems. Good luck is the only advice I can offer

For me the issues are almost always at the edges or near things like for example if you finish with the bridge or the neck/fretboard attached. I think that is the case because it's easy to sand through the finish along the edges, been there done that enough times toe NEVER want it to happen again, and as such I am not as aggressive in final sanding there.

I will say though that since I discovered the 3M polishing papers a few years back they are a God send for cleaning up scratches like that. You could practically finish the guitar to a high gloss with those papers alone.

[Dan of SC]

Originally Posted by Bruce Sexauer

I sand wood surfaces to 220 except endgrain and some other sensitive spots, which I take to 320. Between coats I use 320 except the penultimate coat, upon which I use nothing coarser than 1000. On the final coat I use nothing coarser than 1500 for the last 100 guitars or more, and I have switched very recently to nothing coarser than 2000 grit. The reason for this fanaticism is exactly the issue you describe, Dan.

Bruce, I can't stop thinking about this; if you use 1500 on the next to last coat then 2000 on the final coat you must be spraying pretty near perfect. 2000 is as fine as I go before buffing. I have to start with 4-600 at least to level sand. If I used nothing but 2000 on my final coat it would take me a month to get it level, not to mention 3-4 thousand sheets of sandpaper. Do you thin the last coats? Putting on a really good finish has got to be one of the hardest parts of building a guitar. If I sound like a smarta## I apologize. Just trying to understand. Danny Gray

[Bruce Sexauer]

If you had a copy of my schedule that I have emailed to perhaps 100 people at the moment (I didn't keep track of who) you would know that I used to sand those two coats with 800 then 1200, followed a few years later by 1000 and 1500, and now, a few years later yet, by 1200 or 1500 and finally 2000. Partly this evolution is due to a lower tolerance for sanding marks, partly an increase of patience, and then as you suspect, I am actually getting better at spraying.

The trick with spraying is getting a coat wet enough to flow out to level while not being so wet as to sag on vertical surfaces. Everywhere. Preparing the previous coat is key to success at every stage, as surely we all know. That's why the penultimate coat is sanded as well at my ultimate coat used to be, otherwise it would be relatively challenging to level the final coat with such fine paper.

I will add, just to make you crazy ��, that I use just over 1/2 sheet of 2000 grit paper for that final blocking of the last coat.

[Dan of SC]

Originally Posted by Bruce Sexauer

If you had a copy of my schedule that I have emailed to perhaps 100 people at the moment (I didn't keep track of who) you would know that I used to sand those two coats with 800 then 1200, followed a few years later by 1000 and 1500, and now, a few years later yet, by 1200 or 1500 and finally 2000. Partly this evolution is due to a lower tolerance for sanding marks, partly an increase of patience, and then as you suspect, I am actually getting better at spraying.

The trick with spraying is getting a coat wet enough to flow out to level while not being so wet as to sag on vertical surfaces. Everywhere. Preparing the previous coat is key to success at every stage, as surely we all know. That's why the penultimate coat is sanded as well at my ultimate coat used to be, otherwise it would be relatively challenging to level the final coat with such fine paper.

I will add, just to make you crazy ��, that I use just over 1/2 sheet of 2000 grit paper for that final blocking of the last coat.

Bruce, Thanks for your advice. I assure you it is well appreciated. Danny