dirty checks in sanded finish

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jpford

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Apr 16, 2016
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Hello all, I'm having an intermittent problem with sanding/finishing turned pen blanks. Every so often, I get what seem to be "dirty checks" in a turned piece. I usually turn with sharp tools and sand with 150,240,320, 400. I then use micro-mesh up to 12000. I'm doing both rotational and grain sanding. I also wipe the piece between grits.

The dirty checks seems to happen mostly with light colored woods, curly maple comes to mind. They are not throughout the turned piece, but scattered in no particular pattern. They look like dirt or metal filings have been incorporated into sanding scratches at some point in the process. they simply won't come out, no matter what I do. I've tried to resand, but they will not come out, as if they are impregnated into the wood deeply. I'm sure I'm doing something wrong, but I can't figure out what it is. Are these bad blanks, heat checks from hot drilling, or are they a bad sanding practice?

I'll try to find a picture of what I'm talking about , but I'd appreciate any help if you've seen this before.

Joel
 
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keithbyrd

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I understand - sometimes these things pop up and for the life of me I can't figure it out - but often someone on IAP suggests something and walla!!
So make sure you are not sanding the bushing - metal dust won't come out. Usually see that on the end of the blanks.
Is there anything that can be spraying a fine oil mist - specks? probably not but who knows!
Clean sandpaper - I reuse mirka and sometimes a little powder from a trustone or acrylic blank gets carried over if I don't blow it out.
You may be complicating your problem from too rough sanding? I start at 400 and go up. Can't remember the last time I used 320grit but never 150 on a pen.
Hopefully something will pop up and you will find the answer!
 

jpford

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Ambler, PA
I understand - sometimes these things pop up and for the life of me I can't figure it out - but often someone on IAP suggests something and walla!!
So make sure you are not sanding the bushing - metal dust won't come out. Usually see that on the end of the blanks.
Is there anything that can be spraying a fine oil mist - specks? probably not but who knows!
Clean sandpaper - I reuse mirka and sometimes a little powder from a trustone or acrylic blank gets carried over if I don't blow it out.
You may be complicating your problem from too rough sanding? I start at 400 and go up. Can't remember the last time I used 320grit but never 150 on a pen.
Hopefully something will pop up and you will find the answer!
Thanks for the response! It has to be sanding the bushings. I thought sanding the bushing was unavoidable... And, yes, I usually start around 320 grit, not 150.

Joel
 

thewishman

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Joel,
Sand from the middle of the blank out towards the bushings, then switch the sandpaper to a clean spot and repeat. That will keep the metal out of the wood.:smile:
 

keithbyrd

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If sanding light colored wood I usually switch to delrin bushing the one's used for applying CA to avoid the metal look
 

jpford

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Ambler, PA
Thanks everyone for the replies and tips - Here's an image of what I'm seeing. This is a relatively minor example; I've seen it worse and I've scrapped a couple of blanks and started over. I highlighted the main areas, but I can see the "dirty dust" is more or less over the entire finish.

Joel
 

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Phillip Kelley

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Grain...

I think some of that is simply variation in the grain of the wood...i hav run across similar problems...:confused:



Thanks everyone for the replies and tips - Here's an image of what I'm seeing. This is a relatively minor example; I've seen it worse and I've scrapped a couple of blanks and started over. I highlighted the main areas, but I can see the "dirty dust" is more or less over the entire finish.

Joel
 

jpford

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I think some of that is simply variation in the grain of the wood...i hav run across similar problems...:confused:

You may be right, but this is not the piece of curly maple I started with; It was pristine prior to sanding, and it sure doesn't look like most curly maple pens I've admired...:confused:
 
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jttheclockman

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Joel that is grain variations especially with curly maple because that is what makes it curly looking. When you are sanding basically you can never sand with the grain because it varies. remember you are working with a small piece of timber and you have turned it down even more so it will not look the same as in the raw. When you start your sanding with such a coarse grit you open the pores and are asking for trouble with contamination from the metal bushings. never any need to start with anything less that 220 grit.

For me I am a big proponent for never touching the blank with sandpaper before the first coat of finish. learn to use the skew properly and you will not need sandpaper. I do segmenting work and cross contamination from different colored woods is a huge problem and I solve that with never using sandpaper. Just my skew. Just an opinion.
 

jpford

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Joel that is grain variations especially with curly maple because that is what makes it curly looking. When you are sanding basically you can never sand with the grain because it varies. remember you are working with a small piece of timber and you have turned it down even more so it will not look the same as in the raw. When you start your sanding with such a coarse grit you open the pores and are asking for trouble with contamination from the metal bushings. never any need to start with anything less that 220 grit.

For me I am a big proponent for never touching the blank with sandpaper before the first coat of finish. learn to use the skew properly and you will not need sandpaper. I do segmenting work and cross contamination from different colored woods is a huge problem and I solve that with never using sandpaper. Just my skew. Just an opinion.
Thank you for your response. I'm developing better "skew skills" every day, but have continued my (apparently aggressive) sanding practices. What you say makes perfect sense. I would assume that it's also true of burl or any other-than-straight grain. I'll keep trying... :redface:

"More skew, less grit... more skew, less grit. Repeat"

Joel
 

leehljp

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One other possibility that sometimes happens: If CA is applied and then it is sanded through in spots, then wiped with a cleaner, the spots will darken even when CAed over.

Using a wiping cloth that has metal dust from the bushings will do this, and it becomes more prevalent if wiped after sanding through in spots after the initial CA application.
 

jpford

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Apr 16, 2016
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Ambler, PA
Ran to the woodcraft store today and picked up some hold-fast bushings. Ordered some abralon online and moved all less-than-320-grit sandpaper a minimum of 15 ft away from my lathe station :smile:. Turned a few trinkets using a gouge to start and then picking up my skew earlier than usual. Turned out fantastic. No "dirty dust" and the hold-fast bushings are great!

Now, to work on that CA finish... :redface:

Thanks to you all for your feedback and advice. I continue to be in awe of the wealth of knowledge and spirit of support here.

Joel
 
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