removing swirl marks and scratches from bare aluminum hull

Discussion in 'Metal Boat Building' started by Pen Gwyn, Mar 7, 2020.

  1. Pen Gwyn
    Joined: Mar 2020
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    Pen Gwyn Junior Member

    Hello.
    I am looking for tips and information on proper equipment to finish an aluminum hull.
    At this stage the hull has clear scratches/swirl marks, as you can see in the photo's attached.
    The information I got was to continue with a rotary sander (eccentric) and 50, 80, 120 grit paper.
    We started of with a Festool RO 150 FEQ and 40 grit sandpaper, but this turned out to be too course.
    Almost identical depth scratches/swirls.
    Going directly to 80 grit sianet 150 discs, does not seem to go deep enough for a first step, but would probably do a good job further in the process, before than moving on to progressively finer grit.
    It seems it needs a step in between the current level of finish (from the builder).
    any input is welcome.
    Philippe
    SV Pen Gwyn
     

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  2. Yellowjacket
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    Yellowjacket Senior Member

    Wow, somebody went at that with way way too rough a grit. The problem now is you're going to have to work your way down from where you are now, level by level. In most cases a grit of 2oo or thereabout would be a good starting place, and that is if there were scratches in the surface to start with. What people don't realize is that even if you have some deeper scratches you don't want to start with too rough a grit. If you start too rough you spend most of your time cleaning up the scratches that you put in rather than the ones you were trying to remove. Now that you've gotten it all roughed up, you just have to go down in steps until it's finish you want. If you're going to paint it in the end, paint it with a high build aluminum primer and then sand most or all of that off and then prime it again. If you're going for a polished aluminum finish you have your work cut out for you and you'll just have to step down till it's polished. Also remember that as you go down to finer and finer grits, don't try to get all the scratches that were there, just keep working it and going to lower grit levels. For polishing there are a number of aluminum polishes that will give a great finish once you get down to that level.
     
  3. Barry
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    Barry Senior Member

    Are you painting the hull?
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    That is the key information.
     
  5. Barry
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    Barry Senior Member

    I am hoping that the builder did not do all of that grinding to hide unfairness of the surfaces.
    What is most important for any curved surface is that you use a soft backing disc or you will cut the apex of the curve. Ie you would remove the scratches but introduce flats throughout the shell. If your are not painting it,
    it will not matter much but if you paint the surface with a high gloss finish, you will see the imprints
    Possibly, ( we have never used one) but an air powered sanding board that is extremely flexible would do larger radius curves and follow the contour. I would talk to a body shop about this

    Generally, we used 60 grit for all of our flat surfaces, and when you do the proper build up with primers etc, that was fine. 80 would probably be better. While this seems very coarse for some reason the 60 - 80 grit would create a pretty smooth
    surface. We tried 120 and up, and found that the paper would just clog up. As a test, sand a spot on a piece of scrap, then paint it with GLOSS black spray bomb, wait until it dries and hold it up the light to check smoothness

    For tight welds, if you want to clean these up, Pferd makes a Polyfan SPG curve rigid 5 inch grinding/flap wheel. The out side edge when viewed from the side wraps around the edge, making it possible to carefully smooth out
    inside fillet welds. This is very different that normal flap DISCS They have a few variations, we were only introduced to the Zircon curve but I see now that they have a Alu curve which is supposed to be better for aluminum

    If there are areas that are deeply scratched and you try to remove the scratches just in that area without being able to blend in a an area maybe 5 times to feather out your scratch area, you may end up with a bit of a mess

    So if you are going to paint it with a gloss finish, re evaluate how much you want to take off.

    You said that you tried an 80 but did not get out all the scratches. I would maybe try a 100 first over a large area as this will polish out the finer scratches and give you and indication of where there are deeper gouges rather than
    start with a 60 or 80 and introduce new scratches that you have to take out.
     
  6. Pen Gwyn
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    Pen Gwyn Junior Member

    Hello Barry (and others for fast response).
    The hull was professionally blasted and epoxied below the waterline.
    And no, the hull above the waterline will NOT be painted.
    I want to obtain an even, clean looking surface without scratches, which should then turn to an even dull look, once on the water.
     
  7. Pen Gwyn
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    Pen Gwyn Junior Member

    Thank you all for the feedback.
    I am currently trying to get the procedure right on a testing zone. This is the top of the hard dodger, which will be covered with solar panels later.
    So, any mistakes I make now, will not show later.
    Only when I have the technique down will I be moving on to the rest of the boat.
    And, yes, I understand I have my work cut out for me!

    So, do I understand, that at this stage I may be better off starting with finer grit than the 80, somewhere between 100 (Barry) or even 200 (Yellowjacket), over a larger area.
    And then possibly go back to larger grit, 60 or 80 to smooth out.

    What about type of paper? And speed of rotation?
     
  8. Pen Gwyn
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    Pen Gwyn Junior Member

    Thank you for the tip on the Pferd Polyfin grinding wheel.
    I found the references, and will try this out after the weekend.
    I will also post some more pictures of different zones of the boat, and the type of corrections/finishing that needs to be done.
     
  9. Pen Gwyn
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    Pen Gwyn Junior Member

    Just for the record to all.
    I am confident about the builder (experienced aluminium builders in Holland), and have no doubts about the quality of the build.
    The yard has built many quality boats over the years, and has an excellent reputation.
    The builder did remain elusive however on how to proceed from here. And although I asked the question repeatedly, I never did get any specific information.

    The information on using a rotary sander (eccentric) and 50 (1x), 80 (2x), 120 (1x) grit paper, came from another owner who had his boat built at the same yard.
    I saw the end result when I visited his boat, and it looked OK. Now, I am assuming that the level of finish of the hull would be the same (or very close) for both our boats.
    But my first steps at his procedure are not really convincing. In all fairness, I used 40 grit (did have any 50 available), but I cannot imagine that this would make a tremendous difference.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    I'd look at the mesh abrasives products, which are made of a flexible plastic mesh with embedded abrasive, 3M makes the discs, as do others, it is available in various grades that vary with the aggressiveness of the cut, but I have used it a bit, and it is much better than hard abrasive fibre discs that scar the surface. Cannot remember the name of it ! You just have to avoid sharp projections, which will damage it easily. It is like a supple random mesh about 3/8 or 1/2 inch thick, that conforms well to contours.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    This is the type of thing, you can get discs to fit 7" sanders. Will not gouge.
    3M.jpg
     
  12. gonzo
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    gonzo Senior Member

    I think that you need to consider how much material is needed to be removed to make the hull polished to a shiny finish. It may be too much. Consult the designer or check the plans for the minimum thickness allowed.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    People who can wield disc sanders/grinders without marring surfaces are a rarity, little wonder that many alloy boats are painted even when they don't need to be.
     
    Barry likes this.
  14. gonzo
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    gonzo Senior Member

    It makes no sense that they used a coarse grinder over the whole surface, unless they were trying to cover imperfections.
     

  15. fallguy
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    fallguy Senior Member

    If you started with 40 grit, I'd say you are fairly screwed unless you can afford a significant surface reduction.

    one inch = 24,500 micrometers

    Here are grit rating sizes.
    40 grit about 425
    50 grit about 336
    60 - 265
    80, 190
    100, 160
    120, 115
    150, 100
    220, 66
    320, 36
    400,23
    600,15
    etc

    A Complete Guide to Sandpaper Grit Classification | DoItYourself.com https://www.doityourself.com/stry/a-complete-guide-to-sandpaper-grit-classification

    If you start at grit 40 and work the last progression out and go all the way to 600 grit, you can do the math, but you will be removing 0.070" of material, which is quite a lot. (unlikely to be needed). Starting at 40 was a colossal error to be blunt. Of course, you don't have to remove the full dimension of the grit ratings through the entire set, but you will need to do a grit progression. I would start at around 180 grit in your testing and be careful about grinding the grindings. I have found 180 a good starting point for most aluminum polishing. I use black oxide on metal usually. At some point, I also like to use soapy water on flat surfaces. It seems to cut down on cutting too much. Run from 180; allow some defects from the 40 grit.
     
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