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wet sanding green wood platter; contamination problem

Discussion in 'Woodturning Discussion Forum' started by Steve Shulhan, Dec 30, 2020.

  1. Steve Shulhan

    Steve Shulhan

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    Hi all, I need some advice on how to remove what appears to be sandpaper grit stain from a once-turned, green wood platter. The platter was turned from green arbutus (Pacific madrone) that was cut a few months ago. I wet sanded it using waterproof silicon carbide sandpaper, frequently rinsed in warm water. After sanding with 320 grit, the creamy white, end grain had turned a grey colour. I suspect that the grey colour is from embedded grit from the sandpaper, but I could be wrong...

    If this is sandpaper grit contamination:
    1) what should I do differently next time to prevent a recurrence?
    2) how should I get rid of it? Let the platter dry and sand with regular sandpaper, or...?

    The platter is currently hibernating in a brown paper bag in a cool room. It's almost 3/16" thick and I expect it will move too much overnight to remount and cut a layer off tomorrow. I'm sure that it will warp as it dries, but hope that it does not crack.

    Thanks
     
  2. Mark Jundanian

    Mark Jundanian

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    Others will contribute their opinions, but I doubt that the color change is due to retained abrasive particles.
     
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  3. robo hippy

    robo hippy

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    I turn a lot of madrone. I don't sand it until it is dry. My lathe will go down to about 15 rpm, so I can sand with the lathe spinning. I also made an articulated arm for bowl sanding (I have a You Tube video about it) so that it holds all of the weight of the drill, and I can spin the piece by hand. I haven't had that type of discoloration. Not sure what it could be. Main discoloration I see is spots from metal dust. I do wrap the rim of any madrone I turn with the stretch plastic film, a couple of times around. The edge is most vulnerable to cracks starting. Christian Burshard from Southern Oregon turns a lot of madrone as well, He dries it by putting the piece in a paper bag and putting that inside a plastic bag. The paper bag gets changed daily. Your piece will be dry in maybe a week. I do use a recess, and have a set of pin type jaws. I am able to wiggle a warped recess around a bit and get 4 point contact. You don't need a really secure grip for sanding, especially at very slow speeds. I also seem to get less cracking if I get spring harvested trees than I do when I get mid to late summer trees.

    Oh, if it is metal staining, concentrated lemon juice will remove it. Almost instant when fresh turned, but maybe a couple of applications if the wood is dry. It does tend to bleach dry wood.
    robo hippy
     
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  4. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Never use silicon carbide paper on wood whether wet or dry, but especially not wet wood. It is notorious for what you experienced. The only solution AFAIK would be to turn or sand away the embedded grit. Go with option #2 ... let the wood dry and then use regular sandpaper. I like Norton Pro Sand, but there are a number of other high-quality sandpapers out there.
     
  5. Mark Jundanian

    Mark Jundanian

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    Good to know. I've never heard of this before. But then I've never used silicon carbide sand paper, either.
     
  6. Steve Shulhan

    Steve Shulhan

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    Thanks Reed. I watch and enjoy your videos; I'll look for the one with the drill support.

    I will try using lemon juice on the affected areas; I don't want to bleach out any of the pretty colouring in the rest of the piece.

    It is drying quickly inside a paper bag and has lost 11% of its weight over the last 2 nights. Thanks for the tip about changing the bags regularly. It has already warped a fair bit and I'll have to spin the lathe by hand if I remount it.
     
  7. Steve Shulhan

    Steve Shulhan

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    Thanks Bill. If the lemon juice does not remove the discoloration I'll sand it away when dry. I too, use Norton ProSand; I can source it locally and stock up when it's on sale.

    Is there a wet/dry sandpaper that will not contaminate green wood with grit when used wet?
     
  8. Don Wattenhofer

    Don Wattenhofer

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    I have turned hundreds of translucent goblets in green black cherry and several other species and never experienced any discoloring from wet sanding with silicon carbide paper. GobletCherry2.JPG
     
  9. Steve Shulhan

    Steve Shulhan

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    Beautiful goblet Don. What type of SiC paper do you use, and are there any special considerations for using it wet?

    Here's a photo showing the problem on the underside of the rim at the left and near the base in the centre.
    [​IMG]
     
  10. Bill Boehme

    Bill Boehme Administrator Staff Member

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    I don't wet sand bare wood although I do wet sand thoroughly cured finishes. I use Micromesh for that. I don't know the brand(s) of silicon carbide paper to avoid, but it seems to be the open stock stuff at my local hardware store. It doesn't take much effort to get a black slurry when using that paper wet.
     
  11. Don Wattenhofer

    Don Wattenhofer

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    I just use 3M brand wet or dry purchased at the local hardware store or big box store. The way I do the goblet forms is to do the inside first making sure that the contour is a clean continuous curve without any ridges then wet sand, while the out side is still heavy, with as much water as I can keep in there. The clean up after sanding is first wipe out the inside with lots of water and paper towel to remove all of the wet dust, then clean up the lathe bed. The out side is turned and sanded in stages since it will distort when it gets down to 1/16th" or less so it is difficult to go back.
    Did you check the thickness between the darker areas and the non dark areas? I have found that on a bowl form like yours with all of those features causing distortion that they get thin enough in some spots that it appears discolored. The most difficult part on your piece would be keeping the knots in tact so you should pat yourself on the back for that accomplishment.
     
  12. Bill Boehme

    Bill Boehme Administrator Staff Member

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    I can't say for certain from the photo, but it does appear to be the black grit from the sandpaper embedded in the wood. Here is a 100% crop of the worst contaminated area.

    SiC_grit.jpg
     
  13. Don Wattenhofer

    Don Wattenhofer

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    That looks similar to the tannic acid leaching out in green turned red oak. The local forestry service may have an expert who would take an interest in analyzing the stain.
     
  14. robo hippy

    robo hippy

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    Bill and Steve, looking at that close up, I would think that the spots are metal stains. They will not sand out. There might also be tear out involved, as it looks like that is in the uphill cut area..

    Steve, I am guessing you did it by intent, but I try to cut my bowls near any defects, like knots just because it makes the wood warp differently in that area and gives the bowls a very nice texture.

    robo hippy
     

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