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Thread: End grain rash?

  1. #1
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    End grain rash?

    Not sure what else to call it but what are the best ways other than a sharp tool to combat the rash that you get on end grain when turning dried wood. If you look close you can see it on this kiln dried maple. There is a tiny bit of small tear out but even during sanding I get a hazy looking result that is what I’m calling a rash. Hopefully my question makes sense. Thanks.

    262AAC98-263A-46F1-B04D-743F409AC78D.jpg



    Here are pics of the overall bowl
    9227D6D3-F101-43A3-ABC8-F13F959CE8C0.jpg
    C7D8ADAB-3246-4756-A56C-2E38894E2F87.jpg

  2. #2
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    Other than, as you said, a sharp tool try sanding on one direction only. It may be a bit slow but I usually will only go down to 120 grit. It's not always the opposite direction to how the bowl was spinning. Often I find going perpendicular to the grain is the best way to sand out the tear out. I haven't had an issue in a little while but I wanted to try using sanding sealer first to see if it helps.

  3. #3
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    Thanks Alex. I’m going to try some more tonight to improve that area. Last night I did apply some 50/50 poly and mineral spirits to the bowl. I’ll be curious to see if that makes any improvement to finish sanding.

    Ive run into this numerous times on these kiln dried maple blanks from Rockler. Never fails no matter how sharp of a tool I use I get this result so I’m curious to find out how others combat this effect on dried end grain.



    Quote Originally Posted by Alex Zeller View Post
    Other than, as you said, a sharp tool try sanding on one direction only. It may be a bit slow but I usually will only go down to 120 grit. It's not always the opposite direction to how the bowl was spinning. Often I find going perpendicular to the grain is the best way to sand out the tear out. I haven't had an issue in a little while but I wanted to try using sanding sealer first to see if it helps.

  4. #4
    Greg,
    I too run into this problem a lot and it gives me fits every time....especially since I really don't enjoy sanding for days.
    But if you run your hand over the rash you'll find that it is rougher one direction than the other.
    If you take a heavier grit and hand sand against that roughness until smooth you can then power sand with a lighter grit with greater success.
    ~john
    "There's nothing wrong with Quiet" ` Jeremiah Johnson

  5. #5
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    Thanks John. Hearing you run into this too makes me feel better since you are one of my wood turning hero’s. LOL. Love seeing your work.

    Ill give that that a try and see if I can feel the difference and sand accordingly. Thanks for the tip.



    Quote Originally Posted by John Hart View Post
    Greg,
    I too run into this problem a lot and it gives me fits every time....especially since I really don't enjoy sanding for days.
    But if you run your hand over the rash you'll find that it is rougher one direction than the other.
    If you take a heavier grit and hand sand against that roughness until smooth you can then power sand with a lighter grit with greater success.

  6. #6
    Hero? Yikes I think you might have your bar set a little low but thank you.
    I'm pretty decent with a skew but the fact is, I could sharpen to be in a Gillette shaving commercial and still get that rash.
    Sometimes turning the piece around having it spin a different direction also helps. Either way....it's got to go. The finish will just make it show more
    ~john
    "There's nothing wrong with Quiet" ` Jeremiah Johnson

  7. #7
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    I have a theory I am working on as to why this works, but here's something that has worked for me. First spray the wood with water and let dry. Then start sanding with 400g or 320g, then work your way down to 100g or 80g, then back up through the grits. Make sure to sand slow speed and lightly.

    If anyone tries this, please let me know if it works for you.
    [SIGPIC][/SIGPIC]
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  8. #8
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    Shear scraping with a sharp gouge will take care of your problem. Don't know how deep your imperfections are but as long as your getting "whispers" coming off the tool, you'll get the piece fixed.
    Member Turners Anonymous Pittsburgh, PA

  9. #9
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    I use to have a much bigger problem with this. What i did was practice. First all I would use is my bowl gouge, very sharp, with a fingernail grind. I then started playing around with using the sides of the gouge (vs the tip). I tried to make mental notes but I don't get a chance to turn too often so I have a few pictures with me holding the gouge and arrows pointing which direction (push vs pull) to move the gouge. I'm not an expert (only been turning for a year) but once you figure it out you'll be investing a lot less sandpaper. Also another think to work on is a light touch (still working on that one). I love seeing the wafer thin curls of wood coming off as I turn.

  10. #10
    I love a good theory....so what are you thinking, John?
    Quote Originally Posted by John Beaver View Post
    I have a theory I am working on as to why this works, but here's something that has worked for me. First spray the wood with water and let dry. Then start sanding with 400g or 320g, then work your way down to 100g or 80g, then back up through the grits. Make sure to sand slow speed and lightly.

    If anyone tries this,
    ~john
    "There's nothing wrong with Quiet" ` Jeremiah Johnson

  11. #11
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    Preventing the rash

    I call the rash "micro tearout". Sometimes on problem pieces I'll paint on a coat of thinned shellac and let dry just before the finish cuts. I thin shellac-based sanding sealer with an equal part of denatured alcohol. I think this helps by soaking into the end gran to stiffen/support the fibers, helping by "gluing" short end-grain fibers into place for clean cutting when they might otherwise beak and pull out. Just a theory.

    I've heard that some people use wax or oil for this, perhaps these soften the fibers.

    And of course, sharp, sharp, sharp is vital - after sharpening on a fine CBN I remove the grinding burr and strop gouges on the leather Tormek wheels treated with polishing compound. If I don't polish the edge, I clean up the edge with extra fine diamond hones. If sharp is hard to attain, the Hunter Hercules tool can work like a bowl gouge and doesn't need sharpening (and has the added advantage of deflecting hot chips instead of guiding them down the gouge flute into the hand!)

    Also, following the finish cuts with a sharp negative rake scraper can do wonders. Anything left after that goes away with the hand scrapers I always use as the last step before sanding. I don't power sand so I don't know how to remove the tearout that way.

    JKJ

  12. #12
    I shear scrape with a bowl gouge to eliminate this tear out and am almost always successful, except on wood that's not sound (not the case here; for wood that's not sound, you need to harden the wood with CA, epoxy, wood hardener, etc.). For anyone not familiar with shear scraping with a bowl gouge, see this video starting about 5:20: https://www.youtube.com/watch?v=77Yyd7PGMCA (not recommending this video over others, it was just the first video hit when I Googled "shear scrape").

    In most cases, I can cut the tearout away with a bowl gouge straight off the grinder, but do sharpen right before you make the final cut. In really tough cases you have to hone after the grinder to be sharp enough (but not many) -- just depends on how obstinant the wood is. As someone else said, you want to see wispy long shavings, not sawdust.

    You really want to cut this tearout out, not sand it away, especially on a hard wood like maple. To sand it away, you have to cut all of the wood down to the level of the deepest tearout pit, and that's a lot of wood (and time), especially on something like maple. Much, much more efficient to do that with a cutting tool rather than sandpaper. Although I wouldn't do this with sandpaper, if I was going to sand enough to get rid of the tearout in your picture, I'd be inclined to go all the way down to 60 grit, because you need to cut away a lot of wood. You can't "smooth" your way out of this problem, you have to cut the surrounding wood away, so you stay with your coarsest grit until the tearout (the "rash") is completely gone. Your coarsest grit is for removing wood until there are no defects, then the rest of the grits are just to get rid of the scratches from the previous grits.

    The wood looks "hazy" because there is sanding dust packed into a bunch of small pockets of torn grain. It occurs at that position on the bowl (just on the far side of 90 degree end grain) because that's the area where you are cutting against the grain -- the tool is "lifting" the wood fibers, and they break off rather than being cut off.

    This kind of tearout always occurs to some degree because of the orientation of the cut -- it's just a matter of how visible it is. You might not be able to see it, but it's there. The goal is to cut out enough that either it meets your standard for finish of the piece, or sanding will take care of the rest. While you want to cut most of it out before you switch to sandpaper, it is not uncommon (for me) to find a small spot of it when I go to sand, sometimes because I didn't notice it until I started sanding and that "haze" you describe makes it more visible. If you have just a small stubborn spot, you can often get it out without undo effort by sanding with the lathe stopped, and using sandpaper 90 degrees to the way it is cutting when the lathe is spinning (so work it from the bottom of the bowl toward the rim) -- that's a much faster way to remove it. The idea is to sand to the bottom of the tearout just in that spot, instead of around the whole circumference. You're actually making the bowl a tiny bit out of round, but if you follow by sanding the same grit with the lathe running, it usually blends in enough to become invisible.

    This problem is most pronounced on the outside of bowls because there is less support for the wood fibers when you're cutting against the grain. On the inside of a bowl you also cut against the grain for part of the cut, but because you're on the inside, there is more support for the wood fibers and thus less tearout (usually). A shear scrape with a bowl gouge won't work on the inside because you can't present the tool in the right orientation. In that case you can make a similar cut with other tools. The video I linked to shows how to do it with a scraper. I prefer to do it with either a fluteless gouge, our a large gouge (like 5/8") with a U flute (not a V) without much grind back on the wings (more like a traditional grind than a fingernail grind). Both these tools allow you to rub a bevel but present a long cutting surface at a steep angle. Reed Gray has a really nice video on using a fluteless gouge for this (among other things): https://www.youtube.com/watch?v=suRxCxdMn4k . I guess technically these tools aren't really shear scraping, they're just cutting at a really high shear angle, but the goal and outcome are the same.

    Good luck!

    Dave
    Last edited by Dave Mount; 08-15-2019 at 11:25 AM.

  13. #13
    Nice video. Saw a couple things that interested me in trying....thanks
    ~john
    "There's nothing wrong with Quiet" ` Jeremiah Johnson

  14. #14
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    Rash

    Quote Originally Posted by John Hart View Post
    Nice video. Saw a couple things that interested me in trying....thanks
    I find that the time to deal with rash is before you need to do a finish cut. While you still have plenty of thickness try various methods to find the perfect procedure to turn that particular piece of wood. Try different tools, grinds, sharpening techniques, grain stiffeners ect. Then resharpen and finish cut. If you get rash forget fine SP and go to 100 grit. but only after a shot of RC lacquer to stiffen the grain followed by a sharp gouge. often a 3/8"gouge short beveled bowl gouge will give the best final cut --with a bit of bevel ribbing here.

  15. #15
    I can get rid of most of the rash with my bowl gouge and if needed I run the lathe in reverse and sand out the rest....

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