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-   -   dirty checks in sanded finish (http://www.penturners.org/forum/f28/dirty-checks-sanded-finish-140551/)

jpford 05-04-2016 07:44 PM

dirty checks in sanded finish
 
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

keithbyrd 05-04-2016 07:55 PM

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 05-04-2016 09:32 PM

Quote:

Originally Posted by keithbyrd (Post 1851516)
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 05-04-2016 11:33 PM

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 05-05-2016 05:06 AM

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

jpford 05-05-2016 06:11 AM

1 Attachment(s)
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

Phillip Kelley 05-05-2016 06:32 AM

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



Quote:

Originally Posted by jpford (Post 1851565)
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 05-05-2016 06:44 AM

Quote:

Originally Posted by Phillip Kelley (Post 1851568)
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:

jttheclockman 05-05-2016 08:07 AM

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 05-05-2016 08:14 AM

Quote:

Originally Posted by jttheclockman (Post 1851585)
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


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