Wet-sanding question

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Wet-sanding question

Postby Brian Evans » Sat Jun 23, 2018 2:29 pm

By hand or with a sander? I am working with EM6000 for the first time, and so far it's going according to plan. I have, for the first time, used 3M Open Coat Fre-Cut P600 dry in my quarter-sheet palm sander for the first step, and it went great. Uniform dull surface, no glossy bits anywhere, no visible scratches at all. Now I switch to 800 wet/dry paper, followed by 1200 wet/dry. The word is to not use water, so I have some mineral oil and mineral spirit to mix. My question is should I hand sand as I have in the past, or with my new and very happy experience with the quarter-sheet palm sander should I use that? Obviously not flooding the surface with lubricant, just wiping on. Any thoughts?

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Re: Wet-sanding question

Postby Gordon Bellerose » Sun Jun 24, 2018 8:14 pm

My experience tells me that the sander may be a bit.aggressive.
I always sand by hand and I do use water.
I need your help. I can't possibly make all the mistakes myself!
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Re: Wet-sanding question

Postby Steve Sawyer » Mon Jun 25, 2018 1:37 am

Gordon Bellerose wrote:My experience tells me that the sander may be a bit.aggressive.
I always sand by hand and I do use water.


I seem to recall that you use Brite Tone, Gordon.

My experiments with GF High Performance were wet-sanded with good results. Some here have recommended that waterborne lacquer should never be wet-sanded, and I found something on Crystalac's web site that repeats this caution. Yet you seem to also get good results with wet-sanding.

For me, wet sanding just feels right and I never worry about the sandpaper loading. I'm planning on another test but with Brite Tone, and would like to continue wet-sanding.
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Re: Wet-sanding question

Postby Brian Evans » Mon Jun 25, 2018 7:59 am

The main difference between EM6000 and Brite Tone is that EM6000 is non-catalyzing, while Brite Tone is pre-catalyzed. EM6000 dries and cures, while Brite Tone undergoes a chemical change within several days of spraying where the polymers cross-link to form new chemical bonds. It's a lot like epoxy in that regard, once it's cross-links you cannot chemically bond to it with new coats, but only mechanically bond. One of the touted benefits of EM6000 is that subsequent coats will "burn in" or chemically bond, and in that way is the same as nitro-cellulose lacquer. Brite Tone of course is quicker to use, builds very fast, and is extremely hard and clear once cured, making it a great instrument lacquer. This all from reading the literature and talking to the tech support lines at the manufacturers, I am not a chemical engineer... :)

BTW the guy who said EM6000 should not be wet sanded with water is named Jeff and he is an engineer at Target, who makes the EM 6000 (and other such) finishes. So far I am wet sanding by hand with 50-50 mineral oil and mineral spirits (camp stove fuel, or naptha) and it leaves a lovely finish on both the guitar and my now baby-soft and smooth fingers... Moving to 1200 grit and polish today, in fact.

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Re: Wet-sanding question

Postby Christ Kacoyannakis » Mon Jun 25, 2018 9:00 am

Brian, I am a bit confused by your last post. You said that your research indicated that the Brite Tone chemically bonds itself to previous coats, but EM6000 does not. However, you then noted that one of the touted benefits of EM6000 is that subsequent coats will burn-in, or chemically bond with previous coats, like nitro, which seems to contradict the first. I have used EM6000, and gotten OK results. I have a can of Brite Tone, that I plan to use in a month or two, so I am very interested in the differences. Thanks for your post.
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Re: Wet-sanding question

Postby Brian Evans » Mon Jun 25, 2018 9:46 am

Christ, I's sorry if I wasn't clear. What I was referring to is that Brite-Tone undergoes a catalyst induced chemical change several days after drying where new chemical level, molecular bonds are formed within the applied finish in a process called cross-linking. Once that's been achieved the finish is extremely stable, and will no longer chemically bond to subsequent coats, but will mechanically bond. That catalyst effect takes place several days after it's been applied, so if you follow their instructions and apply three coats the first day, three more the second day and so on you do get full burn in of the coats because the cross-linking has not started yet. I did like 12 coats of Brite Tone over four or five days and had a great result. That's why if you come back after a full cure you need to sand the finish to create the tooth to allow the mechanical bond. It's a pre-catalyzed finish, which means that the catalyst is pre-mixed into the finish before you get it, so it's a one part finish. That means it has a shelf life of some sort, and after it's in the gun has a pot life before it can harden in the gun. Post catalyzed finishes are two part, you add a catalyst while mixing the finish, it normally has a very short pot life (a couple of hours or so) and you apply it very differently in terms of a finish schedule. The EM6000 is non-catalyzed which makes it friendlier to apply, does not cross-link so you can apply finish long (weeks, months) after it has dried and the new finish will still burn in to the old (just like Nitro). You actually can buy a pre-catalyzed version of EM6000, or buy a catalyst that you add, if you want that somewhat harder finish. I said that EM6000 dries and cures, because it does dry very quickly (60 minutes unless the coat is overly wet) and you can recoat after 90 minutes or so, but you are supposed to wait 100 hours for a full cure (without cross-linking) before polishing. This is very similar to nitro-cellulose.

With some post catalyzed car paints I've used, you can't even let the paint dry before the second or third coat (and three coats is all that is normally recommended). You spray a mist coat, let it flash for two or three minutes, spray a wet coat, let that flash for 10 minutes, then spray a third coat if needed. Same with clear coat - one mist coat, then immediately one wet coat, and that's it, you are done. Bake and start wet sanding in 4 hours. Production level car painters are magicians.
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Re: Wet-sanding question

Postby Steve Sawyer » Mon Jun 25, 2018 10:03 am

Brian - are you planning on using the naptha/mineral oil lubricant with the Brite Tone as well?

Also, do you simply wipe down with straight naphtha to clean the surface to prep for subsequent coats?
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Re: Wet-sanding question

Postby Gordon Bellerose » Mon Jun 25, 2018 10:19 am

Steve,

You may have misunderstood me, or I wasn't clear.
I dry sand after every 3 coats while spraying, and only wet sand after the finish is cured in the final stage before buffing.

As far as BriteTone is concerned, I have used it now on 3 different guitars.

I am finding that 9 coats is enough, and possibly could go down to 6 or 7.
The last guitar was 9 coats; some of those were not really heavy coats in my opinion.

I am almost finished that one, and will post some pics once it is complete.
I need your help. I can't possibly make all the mistakes myself!
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Re: Wet-sanding question

Postby Steve Sawyer » Mon Jun 25, 2018 10:37 am

Got it, Gordon. Thanks. I'm more comfortable dry-sanding between coats - it's only a single relatively coarse grit.
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Re: Wet-sanding question

Postby Brian Evans » Mon Jun 25, 2018 10:46 am

If I were using Brite-tone, I would wet sand with water with no hesitation. I'm using the mineral oil mix because that's what the tech rep at Target recommends. I don't have a finish schedule that I am really comfortable with yet, but so far with EM6000 I am not sanding between coats at all, I laid down 8 coats (colour first, then clear), let dry three days, sanded with dry open coat 600 with a quarter sheet sander, laid down two more coats, let dry five days, and now I have again dry sanded with 600 on a sander, wet sanded with 800 with the mineral oil mix, and going on to 1200 wet, then to polish. We will see what we will see... I must say I am really liking the open coat Fre-Cut 3M sandpaper on the EM6000. Extremely fast to level but not aggressive, doesn't clog up, I'm just using a Porter-Cable 1/4 sheet sander. What I do is (with sander running) wipe on an old bath towel to clean the paper, and it seems to last forever. I don't use the oil/naptha wet sand until I am totally done with adding coats of finish. I'm curious to see what happens when I switch to polish, in terms of any residual oil on the surface. I'm just wiping off with a microfibre towel.

As far as I'm concerned, finishing is the true luthiers secret... Black magic until one day it becomes easy, I hope.
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Re: Wet-sanding question

Postby Steve Sawyer » Mon Jun 25, 2018 10:27 pm

Brian Evans wrote:If I were using Brite-tone, I would wet sand with water with no hesitation.

Is this based on discussion with the Crystalac folks?
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Re: Wet-sanding question

Postby Barry Daniels » Fri Jun 29, 2018 10:33 pm

Brian, I too use a towel to wipe off a running wet sander. I found that a cotton terry cloth towel works great. The way I use mine is to leave the towel folded on the workbench and occasionally run the sander over the towel like I am making a sanding pass. Cleans the paper really well.
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