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Technical Question for the Pros - Sanding for Paint Prep - High Spots

Discussion in 'The Hokey Ass Message Board' started by Ziggster, Aug 18, 2025 at 9:08 AM.

  1. Ziggster
    Joined: Aug 27, 2018
    Posts: 2,347

    Ziggster
    Member

    A question I’ve been meaning to ask for a long time is why is it every time I’m sanding down high spots is that the first thing to disappear is the paint around the high spots?
    I’m no professional. Never used a proper paint gun, but have done plenty of rattle can stuff, and 100% of the time I’m always dealing with this issue. Is it my giant fingers, my technique, or is it just the way it is?

    Case in point. Was removing some excess epoxy from my exhaust collector, and I’ve reached bare metal (one top coat), before knocking down the epoxy high spots.

    IMG_9080.jpeg
     
  2. Mostly just thats the way it goes. You either need to knock those high spots down before you continue (the better way) or build everything up around it.

    Hopefully you are sanding with a block of somesort and not just you hand. Your hand is not an accurate backing for sandpaper for trying to make things flat.
     
  3. Ziggster
    Joined: Aug 27, 2018
    Posts: 2,347

    Ziggster
    Member

    Using my thumb/hand. Come to think of it, I rarely use a block or anything like that. Did a patch panel repair on one of my OT vehicle front fenders a few years back. Did use a block on sections of that.
     
    Last edited: Aug 18, 2025 at 11:52 AM
  4. Your hand is a terrible sanding block and should be avoided. It doesn't provide even or flat pressure to the surface.
     
  5. HOTRODNORSKIE
    Joined: Nov 29, 2011
    Posts: 603

    HOTRODNORSKIE
    ALLIANCE MEMBER

    Are you sure it's high spots if you're riding over the top even by hand it can't be a high spot you need to do more hammer and dolly work.
     
  6. Fingers tight together and held at 90 degrees to the direction of sanding is how I was taught for areas where you can’t use a block or long board. If you can feel it with your fingers you will see it in the paint. Your post does look a little helter skelter, long smooth strokes would be where it is at. I learned this at an after school job in a discount gas/body shop, so factor that into how much weight you give this advice.
     
    lostone likes this.
  7. Ziggster
    Joined: Aug 27, 2018
    Posts: 2,347

    Ziggster
    Member

    High spots for sure. Gobs of hi-temp epoxy stuck to the surface of 4” round exhaust collector tube, but it doesn’t matter. Gage the sane issue with body filler. Used some body filler many a time to fill in a low spot, and end up sanding area around high spots first. I need to watch some auto body vids on this type of stuff.
     
  8. manyolcars
    Joined: Mar 30, 2001
    Posts: 9,564

    manyolcars

    it looks like that paper is too coarse
     
  9. Over sanding is the biggest hurdle for beginners
    Under applying filler is another.

    some high spots can be due to the nature of the dent. Like tossing a rock in mud, you get a ridge around the low area.
    To cure that, tap or pull the low up, tap the high around the edge down.

    your “high” areas look to be the result of both under applying filler and over sanding.
    As the metal around the dent begins to appear near the dent, that indicates that that area is either a high spot or you are done sanding in that area.
    if you reapply filler to those spots, sand with the contour of the part. If the surrounding metal starts to show up and the filler indicates it’s still low, stop sanding. Scuff up the low spot and apply more filler.
    If when using your hand to check for level and the surrounding exposed metal feels high, tap it down than apply additional filler.
    Apply enough to the area that allows the filler to feather into the metal. In other words, wipe an area slightly larger than the dent.
     
    Last edited: Aug 18, 2025 at 5:03 PM
    winduptoy, guthriesmith and squirrel like this.
  10. squirrel
    Joined: Sep 23, 2004
    Posts: 59,289

    squirrel
    Member

    Might help to apply filler to the entire area, well past the edge of any dings or waviness. Then sand with a block (maybe 60 grit?), until the filler starts to get thin enough you can see through it in a few places, and STOP sanding it. If there are obvious low spots that the sandpaper didn't contact , apply filler again over the ENTIRE area, and repeat the coarse block sanding.

    When you're happy with the filler, then prime with high fill primer, and again sand with a block (maybe 150 grit?) until it starts to get thin in a few places, and STOP sanding. Then prime again with high fill primer, and sand (with 240 or 320?) until the primer gets thin in a few spots.

    This is kind of how I do it, and it usually only takes a few tries to get it almost good enough to paint.

    Have fun...
     
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  11. Ralphies54
    Joined: Dec 18, 2009
    Posts: 794

    Ralphies54
    Member

    Epoxy filler is probably a lot harder than rattle can primer and would disappear a lot faster than the filler when hand sanded.
     
  12. As @daylatedollarshort mentioned it looks like you are just sanding at random all over the place. You will never get anything flat that way. If it was me I would use a long flat sanding block and sand up and around the tube. The blue line being your sanding block and the yellow the direction of movement around the tube from bottom of picture to top. Then repeat from the opposite side in the opposite direction or from the same side but top to bottom..
    IMG_9080.jpeg
     
  13. Ziggster
    Joined: Aug 27, 2018
    Posts: 2,347

    Ziggster
    Member

    In my case, I would tend to agree with @anthony myrick In terms of over sanding and not applying enough filler generally speaking.
    The example in the pic I chose, was probably not the best to illustrate my question. To be clear, it’s a 4” dia exhaust collector that I’ve been working on. I applied some hi-temp epoxy around my welds that were contoured to fill in some imperfections. The spots in the pic were various small nicks, dents, and scratches where I applied the epoxy. Used 80, 120, and 220 grit.
    In this case, on something that is round what would you suggest? Also, had the same issue sanding the epoxy around the welds for the inlet tubes. In those areas, there is no space to use any tool. So, basically small bits of sand paper rolled up, and using it almost like a toothbrush. First pic after first top coat, and another application of the epoxy. You can see all the bare metal surrounding the areas where I applied the epoxy. Second pic, finish sanding the epoxy before first top coat. Used VHT caliper paint.

    IMG_9081.jpeg
    IMG_9071.jpeg
     
    anthony myrick likes this.
  14. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 6,000

    2OLD2FAST
    Member
    from illinois

    I can't imagine any filler other than maybe lead holding up to the heat of an exhaust system ?
     
    klleetrucking likes this.
  15. Ziggster
    Joined: Aug 27, 2018
    Posts: 2,347

    Ziggster
    Member

    Haha! We’ll see. There is about an 18” flex tube between the manifold on the engine and the inlet to the collector. I think the max temp of the epoxy was around 500F. If it falls off, I’ll be disappointed, but overall not a big deal. Just my ADHD kicking in overtime. lol!

    IMG_9029.jpeg
     
    winduptoy likes this.
  16. To get something flat you need to sand it as a whole not just here and there where you add filler. You will chase your tail forever doing that. That can't always be done but the section shown in your original picture you could.
     
    Ziggster and winduptoy like this.

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