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Technical Final Sanding

Discussion in 'The Hokey Ass Message Board' started by blowby, May 21, 2019.

  1. blowby
    Joined: Dec 27, 2012
    Posts: 5,765

    blowby
    Member
    from Nicasio Ca

    How fine do you sand your cars before paint? Same for enamel, urethane, single stage, clear coat?

    Just as I thought I was getting close to booth time my pal came over and said I'd better get all those sand scratches from the bodywork cleaned up. Disheartening, but he is right of course. A year or two ago I painted my work truck single stage white urethane and was surprised how much sand scratches showed up. He said in the Centari days it flowed more and would fill more imperfections than urethane, which tacks up quicker. Seems to be the case from what I remember.

    I'll be using single stage urethane again most likely, and have it sanded with 320 now. Good enough? What about sand scratch swell, is that a thing of the past with today's urethane primers and topcoats?
     
  2. Tim
    Joined: Mar 2, 2001
    Posts: 9,337

    Tim
    Member
    from Raytown Mo

    300 seems petty standard
     
    blowby likes this.
  3. azmodela
    Joined: Mar 2, 2010
    Posts: 23

    azmodela
    Member
    from Tucson, AZ

    I wet sand to 600 before paint. I do the same for all types of paint.
     
  4. john worden
    Joined: Nov 14, 2007
    Posts: 1,403

    john worden
    ALLIANCE MEMBER
    from iowa

    Sand scratch swelling or shrinkage is definitely not a thing of the past.
    400-600 grit range is recommended for most top coats (color or clear)
     
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  5. When you take it to 600 before paint,,,
    Any imperfections shows up before it gets expensive and you have to live with them.
    Single stage solid color you can fix them sorta
     
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  6. Oldioron
    Joined: Dec 12, 2018
    Posts: 574

    Oldioron
    Member

    It really depends on if your using a sealer or not and if your wet sanding or dry sanding.
    I use 320 on a finnish DA dry first on large panels then 320 by hand if a sealer is used and 500 wet if no sealer. This is for single stage.
    Two stage I still like wet sanding but I use the 320 DA first then 500 and not the P paper.
     
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  7. anthony myrick
    Joined: Sep 4, 2009
    Posts: 4,207

    anthony myrick
    Member
    from al

    320/400 on old school enamel
    400 on single stage urethane
    500 on solid color base/clear
    600 for base/clear metallics. especially light colors
    may go to 800 for metallic colors if using water based
    the thinner the material. the finer the scratch

    this is a generic list
    paint product tech sheets will inform you on surface prep. I tend to always finish with the finest grade paper listed. A scratch that does not exist can not come back to bite you
     
  8. Slopok
    Joined: Jan 30, 2012
    Posts: 2,194

    Slopok
    Member

    Any scratch that you can see will not be hidden by paint but rather magnified!
     
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  9. Blue One
    Joined: Feb 6, 2010
    Posts: 9,568

    Blue One
    ALLIANCE MEMBER
    from Alberta

    I went with 400 for Single Stage acrylic urethane.
     
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  10. oldolds
    Joined: Oct 18, 2010
    Posts: 2,680

    oldolds
    Member

    The product spec sheet should tell you what is recommended. Different paint systems want different things. Some paints chemically bond with the undercoat and can be sanded finer. Older style paints like enamels are surface adhesion they need the 220-400 grits for adhesion. I would put most single stage urethane with these
    Silvers need to be scuffed a bit finer than other colors because the darn metallic can collect in an odd sand scratch in the primer and look like a scratch that will never buff out.

    When I first did clear coats I was using NAPA paints, early 90's. Their two part primer was self healing, meaning that if you scuffed it a week ago the pores in the primer would heal and you had poor adhesion, It needed to be scuffed within 24 hours of applying paint. So that is something else to consider. I have not done much painting in the last 15 years, so things have probably changed
     
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  11. anthony myrick
    Joined: Sep 4, 2009
    Posts: 4,207

    anthony myrick
    Member
    from al

    Good point
    I teach my students to re scuff if not painted within 24hrs after a final sand. Especially after wet sanding.
     
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  12. LM14
    Joined: Dec 18, 2009
    Posts: 1,496

    LM14
    Member
    from Iowa

    My painter literally just left my shop. We spent a large part of the afternoon talking logistics and where he wants me to stop. We’re using single stage on my coupe and he went over everything with his hands and pointed out 2 areas he would like to see more work by me. I’m starting with 120 and taking it to 320 grit dry. When I’m done and he’s ready it will go to him in my primer over that 320 work I’ve done. He will lightly block the whole body again and fix any issues that surface before applying his high fill primer. He’ll block again down to 500 before shooting sealer and color.
     
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  13. anthony myrick
    Joined: Sep 4, 2009
    Posts: 4,207

    anthony myrick
    Member
    from al

    3m makes these now for all overs
    ABA56CD5-E14E-484E-B144-66C8B199D363.jpeg
     
  14. BigChief
    Joined: Jan 14, 2003
    Posts: 2,036

    BigChief
    Member

    Almost without exception I've used 3M fine pads (I think they're gray) as a final go over everything just before the final wipe down, tack rag and sealer coat. They're nice to hit your panel edges and seams with....knock down the occasional sharp line at the panel edge that got missed and give everything a tidier look.

    Sent from my SM-G950U using The H.A.M.B. mobile app
     
  15. john worden
    Joined: Nov 14, 2007
    Posts: 1,403

    john worden
    ALLIANCE MEMBER
    from iowa

    Be sure to clean carefully after using the pads, or anything for that matter, as they produce debris.
    I final clean with PPG DX 320 Fast Evaporating Cleaner now known as PPG SX 320
    When used according to directions leaves no residue.
     
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  16. for a down and dirty paint job always used red pads they equal to 320 grit wet with dish soap in the water..

    for the better jobs 400 wet dry wet of course and grey pads for the edges.. the grey pads are equal to 400 grit this would be for 2 stage paint jobs, for fine metallic paint 600 wet before top coats..

    Ask 1000 different people and you will get 1000 different answers on what they use.....

    First guy I worked for painting all we did was hand rubbed lacquer paint
    3 coats sand it 3 coats sand it 3 coats sand it 3 more sand hand rub it with used cloth baby diapers

    I tell you I DONT miss those days....
     
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  17. The37Kid
    Joined: Apr 30, 2004
    Posts: 25,459

    The37Kid
    Member

    If you have spent all the time wet sanding a dead flat NO orange peal primer surface, how do you manage to spray a coat of sealer without orange peal that doesn't need to be blocked dead smooth?
     
    Last edited: May 27, 2019
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  18. anthony myrick
    Joined: Sep 4, 2009
    Posts: 4,207

    anthony myrick
    Member
    from al

    1 way is to use a good sealer that lays down smooth. The 4:1 urethanes and 2:1 epoxies usually spray with some minor texture. The 1:1 epoxies lay down nice.
    Another way us to spray the epoxy and let it sit overnight then lightly sand
    I prefer this because any trash can be removed from the sealer
    The peel in a sealer is negligible anyway.
     
  19. The37Kid
    Joined: Apr 30, 2004
    Posts: 25,459

    The37Kid
    Member

    THANKS! Once someone puts color over primer/sealer with orange peal it can never be removed. You may block the paint perfectly, but viewed at a certain angle the orange peal is there.
     
    Last edited: May 28, 2019
  20. Primer isn’t sanded because there’s orange peal,,, primer is blocked and that fills imperfections. The thing is that any peal texture comes off first.
    Getting texture in primer kinda pisses me off too. Nothing really happens until the texture is gone and that’s wasted effort and materials and
     
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  21. anthony myrick
    Joined: Sep 4, 2009
    Posts: 4,207

    anthony myrick
    Member
    from al

    Only on a base clear
    Light color metallics will magnify any irregularities big time
     
  22. Oh yeah, it’s like cellulite under the clear.
    My buddy calls it flutter.
     
  23. Trying to pick up some tips here, but for me (and others not in the U.S.) could someone explain the types of paint- e.g. is "urethane" what we would call "acrylic"?- 1 part (no hardener), goes off quick, and can be cut after a week or so? - sounds like the stuff that everything was painted with 20-30 years ago.
    And what do they call the 2part isocyanate stuff ? (We just call it 2 pack)
    I think Enamel is just Enamel, takes a while to go off (Dry), leaves a good gloss straight from the gun. (called "Jam" here sometimes.)
     
  24. The37Kid
    Joined: Apr 30, 2004
    Posts: 25,459

    The37Kid
    Member

    That's it, I've worked on paint jobs that had it after someone else did all the spray work and all the cutting and buffing will never make it disappear. Bob
     

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