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Sanding Scratches

Discussion in 'Getting Started' started by Dave Fritz, Apr 7, 2018.

  1. Dave Fritz

    Dave Fritz

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    There are times when after completing a bowl I find sanding scratches that I hadn't noticed before. I know it's best to use a low light to rake the surface to expose scratches but I'm not sure how to really easily do that. Do you have examples of how you check for sanding scratches while the object is still on the lathe?
     
    Last edited by a moderator: Apr 7, 2018
  2. odie

    odie

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    Dave......make sure you get several light sources, and have the light shine from different directions. A combination of incandescent, LED, and fluorescent lights work for me. :D

    -----odie-----
     
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  3. Mark Wollschlager

    Mark Wollschlager

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    Those late emerging scratches are my bĂȘte noire.
    I employ a as many tricks as I can to eliminate them.
    As far as checking goes I will wipe the piece with mineral spirits after dusting and potentially using compressed are to blow the surface.
    This helps to dislodge sanding dust that has been packed into scratches.
    Using the light source after this gives me a clearer view.
    As you get into higher grits, the sanding dust becomes so fine that it can mask scratches if not cleaned off.
     
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  4. John Torchick

    John Torchick

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    Good advice, Mark. I used a can of compressed air that you use for cleaning computer keyboards and electrics. I got an air compressor for Father's Day so that stays close to the lathe. I blow off dust after each sanding phase.
     
  5. Owen Lowe

    Owen Lowe

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    When did you last visit an optometrist? ;) (Not really being all that silly.) I find that I need to examine surfaces very carefully to pick out scratches remaining from previous grits. If you polish the surface with 600 grit or higher, the scratches will often stand out. The burnished surface can then be pulled back a few grits once the undesired scratches are removed prior to finish.

    An auto body painting technique is to apply a fogged coating of contrasting finish and sand that off. The low spots will retain the contrasting finish so you know where to spend more time. I wonder if a shellac wash coat or some such could mimic this technique.
     
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  6. Mark Wollschlager

    Mark Wollschlager

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    Another source of late emerging scratches can be 'contaminated' sand paper.
    Introducing scratches later in the process is not just a possibility.
    Think of them as sanding catches ;)
     
  7. Mike Johnson

    Mike Johnson

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    Rubbing the wood with a little charcoal on the fingers and lightly sanding the piece would most likely reveal scratches. When I finish Lichtenburg pieces after rinsing and scrubbing the charcoal out of the designs and then sanding the wood lightly, scratches left behind in the prior sanding process jump out quickly from the charcoal that catches on the rough grain in the bottom of the scratch.
     
  8. john lucas

    john lucas AAW Forum Expert

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    I fight it also. I use a light with a small filament if I can. That is more of a point soarce and shows scratches better. My light is mounted on a goose neck so I move it all around as well as moving the piece looking to see if I've sanded all the deep scratches. Still sometimes I don't see them until I get to 320 or 400 and then of course have to back down to what I think caused the problem. So your not alone. I will often sand with the grain after my initial sanding. This shows up cross grain scratches. I wipe the piece down between grits. Basically everything mentioned above. I do have trifocals and I blame that. :)
     
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  9. Gerald Lawrence

    Gerald Lawrence

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    As was suggested check your eyes. I thought I was doing pretty good until I got my cataracts removed....Wow what a lot of scratches. Some of the scratches do not show till I get to 800 and then I back down to 220 and start up. I used to try to sand the complete piece and then a friend said he sands the problem as if the shape is changed slightly no one can tell except me.
     
  10. Dave Fritz

    Dave Fritz

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    Thank you all. You raise a good point John. It's when I take the piece into different light that I notice the scratches. The same eyes see it so I'm going to assume it's the light that has changed. The problem is the different light is off the lathe, I'd like to be able to examine it while on the lathe. Maybe a flashlight at different angles?
     
  11. john lucas

    john lucas AAW Forum Expert

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    It's been well documented that sunlight causes scratches. Just ask any turner. You sand it to perfection inside and then take it outside and there they are. So just keep it in low light and the sun can't cause those scratches.
     
  12. Dave Fritz

    Dave Fritz

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    At times it seems like that's the case John. It appears it's a problem for everyone.
     
  13. robo hippy

    robo hippy

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    Another variation on what John said, from some one over at WTR: "Never take a finished piece into the house from the shop on a sunny day. Sunlight causes scratches." Been there, done that, more than once.... So, how to get them all out??? You have to be able to see them first. This involves good glasses and proper lighting. As far as glasses, the cheap reading glasses are about a 5 on the 10 point scale, unless you just barely need them. Prescription glasses made a huge difference for me. As for lighting, I prefer the light spectrum of the 'therapy' lamps, which are almost all florescent types rather than the LED lights which all seem to be in the white spectrum. Our eyes evolved to see in natural sun light. Ott lamps are one that all the needle point and hand quilters use. Most of those lamps are not bright enough or don't cast a broad enough beam for bowls, but for smaller pieces would work fine. I have been using Blue Max floor lamps, also used by hand sewers, which have a 65 watt (I think), 3 long loops type of bulb in them. I can see just about every thing with them. These are also good for grey rainy days in western Oregon when you need a little sun light fix... Other than that, it is being able to tell when you have all of the scratches from the previous grit sanded out. You will also spend more time sanding with the coarser grits than you do with the finer grits. I avoid 80 grit when ever possible, which now is most of the time. If I do need it, it is because I have turned some thing that had patina to it as it dried, like sap wood on cherry, and I have to sand it down to white. I follow that with 100 grit. Some times the 80 grit marks are harder to remove than the tool marks... After 120 grit, I start to try to make a contrasting scratch pattern. So, on the outside of a bowl, and with the power sander/drill, I will sand using the disc on the 10 o'clock part of the pad, and then next grit switch to the 2 o'clock part of the pad so the scratch marks will criss cross each other. I use a firm/hard pad up to about 180 or 220 grit, medium pad up to 400, and if I go over that, then I switch to a soft pad. I never blow off a piece when sanding. In part, I don't want to spread dust around. There are some who proclaim that they do it to make sure there are no remaining grit pieces stuck in the wood, which cause the 'mystery' scratches, which seems to explain why they turn up... Well, instead of blowing out the bowl, I wipe it off with my hand, usually in the 180 grit stage. Out of all the thousands of bowls I have sanded and hand wiped, I have never felt a single grit of abrasive stuck in the wood. I am guessing that the mystery scratches come from not getting all the grit scratches out from the previous grit before stepping up to the next grit. Hand rubbing the bowl also seems to push the finer dust into any remaining scratches, which actually high lights them rather than hiding them.

    One other cause of mystery scratches can come from the discs and the pads. If I use a 3 inch disc on a 3 inch pad, even if it is perfectly centered, the plastic backing for the hooks can leave 80 grit type scratches. Same if I am using still paper or cloth discs. The edges are hard and don't flex, and especially on the inside of a bowl, they can leave deep marks of the "hey, that wasn't there before I started with the last grit, what happened???"

    Of course, some times we just aren't paying close enough attention...

    robo hippy
     
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  14. John Torchick

    John Torchick

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    I prefer to look at my turnings in complete darkness. The scratches disappear! robo hippy, I appreciate your comments. Could you expand on hand wiping? Bare hand or do you use a cloth of some kind? Tack cloth? I keep some old towels in the shop. Yes, I have used a light puff from my air compressor. Your comment are most welcome.
     
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  15. robo hippy

    robo hippy

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    Bare hand. I finish, with the Doctor's walnut oil, and my hands usually have a minimal bit of oil on them, so that picks up some of the dust, but it seems to work the fine dust into the scratches. I think if all of the dust was picked up, then they wouldn't high light as well, or maybe I did get all the scratches out..

    robo hippy
     
  16. Emiliano Achaval

    Emiliano Achaval Administrator Staff Member

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    I usually do my oiling outside, only to go right back inside to get rid of some scratches that the sun revealed!!
     
  17. Emiliano Achaval

    Emiliano Achaval Administrator Staff Member

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    I always enjoy your postings... Find them very interesting, you are not just a turner but a keen observer... Aloha
     
  18. Steven Forrest

    Steven Forrest

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    This is a really useful thread, all. To reinforce some of what has been said above: daylight reveals stuff that I never, ever see no matter how I light it in my shop. I finally learned that patience was a key part of getting the scratches out - I need to wait until I can actually look at the thing in the daylight before proceeding to finishing. (As a hobbyist, I'd want to finish the damn thing and have something to show for all my hard labor!) Raking light is key - the light source needs to cast shadows. Your eyes (again, glasses as needed, magnification is even better) will note very fine scratches if you can cast the shadows. One thing I learned from watching David Marks (he's local, and I have helped out in his shop a bit) is that he has one of those lights with a magnifier built in. Killing two birds with one stone. Finally, humbly differing from Robo Hippy, who I respect greatly for the depth of his explorations, when I got compressed air in my shop, it was a revelation. It gets dust out of scratches, pits, etc. that you might not ever notice until applying the finish.
     
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  19. Bill Boehme

    Bill Boehme Administrator Staff Member

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    Without a doubt, that has to be it. Dust is the best thing in the world for hiding scratches.

    In addition to what John said about sunlight causing scratches, compressed air and shop vacs are potent accomplices of sunlight in causing scratches.
     
  20. Mike Johnson

    Mike Johnson

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    Sanding scratches is a lot of work, it is easier to remove them first before applying a finish :) Hard to get your sandpaper in the bottom of the scratches.
     
  21. odie

    odie

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    When you do notice a scratch that should have been taken out with a previous grit, it's often best to go down 1x grit, and hand sand in a direction that is opposed to the direction of the grit you were using while the lathe was spinning......then go back to the grit you are currently at. Not only will an opposed direction work, but you can also hand sand in an orbital motion.

    -----odie-----
     
  22. Emiliano Achaval

    Emiliano Achaval Administrator Staff Member

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    Today, the Koa bowl that I finished at first glance looked flawless. If you look at the bowl straight, like on a shelve, it appears perfect. Take it outside, tilt it at about a 45 degrees and you can see some very fine scratches on one very small area... I just hope they keep inside the house and not outside! LOL Sanding is my favorite least thing to do, but perhaps one of the more important tasks, something that would make or brake a bowl... Since I hate sanding so much, I have spent decades trying to master woodturning tools so I don't have to sand. I think that I need another lifetime, maybe a few lifetimes before I can achieve what I'm looking for, to master all tools...
     
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  23. john lucas

    john lucas AAW Forum Expert

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    Ditto. Been turning for 35 years and once in a while I get a piece that only needs very light sanding. Don't think I've ever turn a piece that doesn't need sanding.
     

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