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Hot Rods Question on applying Bondo

Discussion in 'The Hokey Ass Message Board' started by blazedogs, May 9, 2015.

  1. blazedogs
    Joined: Sep 22, 2014
    Posts: 362


    Every time I sand bondo after applying it I( undercut it) creating a pocket where the bondo is.This usually occurs on curved surfaces.The bondo of course sands faster and easier than the metal .When I'm done and apply the finish coats on I can see waves and the flaws I know the answer by most body men is "it takes practice"

    Any tips or ideas to help me? Gene
    Model T1 likes this.
  2. mr.chevrolet
    Joined: Jul 19, 2006
    Posts: 6,358


    no, it takes more Bondo.
    belair and Model T1 like this.
  3. blazedogs
    Joined: Sep 22, 2014
    Posts: 362


    I guess I should have added ,it looks and feels perfect until I apply the finish coats
  4. what are you using for sanding board?
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  5. Jethro
    Joined: Mar 5, 2001
    Posts: 1,401


    Your hands don't see as well as your eyes. I call it bodywork denial. It feels ok and looks ok but when you topcoat it looks like the ocean. I use wax and grease remover after I fill. I wipe a wet coat of it over the panel and stand back at different angles to look for the waves. The shine when it's wet simulates paint. Works for me.
  6. Dick Stevens
    Joined: Aug 7, 2012
    Posts: 2,569

    Dick Stevens

    Sorry, it does take experience and with experience you can feel things your eyes can miss. Try not trying to get it quite as close before shooting primer and be ready to sand and have to reprime, I have used different colors of primer in different coats to help see where you are in getting it ready for finish coat.
    belair and Sgt. John like this.
  7. tb33anda3rd
    Joined: Oct 8, 2010
    Posts: 15,032

    1. Connecticut HAMB'ers

    spread the filler out beyond the low spot, sand using coarse paper [40grit] when you see the 40 grit scratches hit the metal along the outer edges of the filler, stop. change to a finer paper [80/100], and again ,when you see it starting to sand through to metal on the outer edges, stop. spray the filler with any color paint LIGHTLY then sand some more. this step will show any low spots because the paint will stay in any valleys or divots. fill if necessary, repeat. if no low spots finish sand with 180/220 and if done right you should sand out any other scratches from the previous grits. there should be no abrupt bondo line, the outer edges should be thin enough to see the metal through.
  8. dcs13
    Joined: Sep 28, 2010
    Posts: 92


    Use a quality filler and you will find your work to be easier and better results. Bondo brand is not a quality filler.
    loudbang, Beau and gimpyshotrods like this.
  9. 1pickup
    Joined: Feb 20, 2011
    Posts: 826


    I haven't found it to be helpful, but many say put a soft cloth (old t-shirt, etc) under your palm when you run your hand over the area to check for waves. My guess is it takes away the different surface feels, & lets you concentrate on the flaws.
    When I think it's good, I'll spray a WET coat of primer on & look at it in good lighting. Even if that means holding a trouble light in one hand while spraying. That gives the closest look to shiny paint, & magnifies the flaws.
    loudbang and belair like this.
  10. KJSR
    Joined: Mar 7, 2008
    Posts: 2,135

    from Utah
    1. Utah HAMBers

    Sounds like you need a longer board.......
    clem and JimSibley like this.
  11. Buddy Palumbo
    Joined: Mar 30, 2008
    Posts: 3,857

    Buddy Palumbo

    A body guy told me (and I feel it actually stands true) - when feeling your repairs to see how well you sanded it ... Use the opposite hand from your writing hand. In other words , if you write with your right hand , feel the repair with your left hand. Sounds dumb , but it works !
  12. BobMcD
    Joined: Jan 25, 2013
    Posts: 322


    For me, I've gotten best results using a Durablock and sand in a criss cross pattern not in a straight line.
    belair likes this.
  13. oldsroller
    Joined: Jan 3, 2007
    Posts: 119

    from PA

    Use a long durablock or softsander (these work great on curved surfaces) and definitely sand in a criss-cross pattern. And another thing that works well ,when checking for straightness, is to either close your eyes or look away and then run your hand across, sounds silly but it works. Your eyes can't convince you its level when you are not looking at it. I do both things pretty much daily repairing wrecked truck parts.
    loudbang and Kiwi 4d like this.
  14. Autodave
    Joined: Jul 28, 2013
    Posts: 125

    from Menifee,Ca

    Do you have a picture of your repair? Are you undercutting it or getting flat spots?
  15. And when you are running your hand over the area, Take long swipes a few inches or more before and after the repair area.
  16. odass
    Joined: Mar 19, 2013
    Posts: 127

    from se ok

    Paint stick with sand paper works best for me when working allot of curves, been sanding on this damn 41 Chevy forever it seems
  17. gimpyshotrods
    Joined: May 20, 2009
    Posts: 16,143


    If it says Bondo on the can, it belongs in the dumpster.

    Use real filler.
    Beau, da34guy and thirtytwo like this.
  18. Kiwi 4d
    Joined: Sep 16, 2006
    Posts: 2,487

    Kiwi 4d

    Durablock sanding boards do not lie , we have found these to be excellent in a variety of shapes.
  19. Gary Watson
    Joined: Mar 15, 2014
    Posts: 2

    Gary Watson

    Use a high density filler, eg 4L can you can tell by weight, the heavier generally the better, manufacturers of cheaper filler whip it up and it adds air to the mix, it should be firm when it comes out of the can.

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  20. indyjps
    Joined: Feb 21, 2007
    Posts: 3,576


    I don't know about that, I use bondo brand to fill cracks in my garage floor so the creeper doesn't get snagged, even takes topcoat well (xylene based garage floor paint)

    Blazedogs, all of the above advice should be followed. I couldn't convince my father that bondo and rattle can primer was shit. He had to sit and watch the filler shrink, and primer die back for the first 4 years after the car was painted. It was very straight year 1, it looked like shit 5 years later, garaged always, never rained on.
    Last edited: May 10, 2015
    gimpyshotrods and Beau like this.
  21. 1great40
    Joined: Jan 1, 2008
    Posts: 445

    from Walpole MA

    The correct sanding block and a criss cross technique are key. Unfortunately, it does take practice and lots of it. And while a lot of you nay sayers will do anything to bash the Bondo product, you should note that the Bondo name has been owned by at least 3 different manufacturers that I can remember and it's doubtful that the formula has remained consistently rotten. Certainly there's nothing wrong with using a professional grade polyester filler but that is not the OP's problem here. He never said that his panels were coming out great and then dying. He has to build experience and technique.

    The most important thing I ever learned about bringing a panel to shape was a term I got from a guy in the lens grinding shop when I worked for Polaroid. He told me "figure, then finish". Many novices try to sand the filler to a finished texture and in the process, ruin the shape, or the "figure" of the panel. He needs to work the figure with a coarse grit paper on the longest board possible for the area that he's fixing. Once he is confident in the shape, then you can either, skim the panel with a little more filler and work it with a finer grit, or spray it up with a high build primer and block it from there.

    Another thing that usually bites a novice is that their initial metalwork may not be sufficiently shaped. It's not unusual for a damaged area to have a disturbed area around it that is actually raised. You need to be sure that you're not trying to to fill a panel where the existing metal is not in the correct plane to begin with.

    Practice, practice, practice and use the wet techniques that have been mentioned. Another really easy way to assure smooth panels is to employ a broom stick or other straight, long item when viewing your work. If you're working in wet primer, before it flashes and goes flat, hold the broom stick about a foot or two away from the panel and view the reflection of the stick while moving it around. A nice smooth panel will show a perfect reflection of the broom stick if it's flat. A curved panel will show a graceful arc shaped reflection of the broom stick. A reflection that shows an interruption of the smooth reflection shows the area needs more work.

    Try the broom stick on an undamaged shiny panel of any car. Learn how to read it. The reflections don't lie.
    Last edited: May 10, 2015
    cretin, pitman, Squablow and 3 others like this.
  22. Buddy Palumbo
    Joined: Mar 30, 2008
    Posts: 3,857

    Buddy Palumbo

    I like the broomstick idea. Will have to try that someday.
  23. odass
    Joined: Mar 19, 2013
    Posts: 127

    from se ok

    I also will try the broom stick. Good info here, I found long time ago that as stated above that the main reason for filling not working out is because of the metal shaping. Takes allotto get metal shaping down, at least for me it did​
    Joined: Jun 3, 2005
    Posts: 7,720


    A friend of mine for too long to admit calls it "cat whiskers". When you see them appear at the edges of the work STOP SANDING and check what you need. The missing element is what's called "feathering", or where the 2 different surfaces meet. You can help yourself along the way by spraying some aerosol primer over the repair as a guide coat. When your sanding is taking it all off on the metal and filler evenly (2 different surfaces) then stop and check the work. You'll be surprised at how soon it's done. The refinish world is rife with over-sanders. Just eyeball some shiny cars at shows and you'll see it.
    pitman likes this.
  25. belair
    Joined: Jul 10, 2006
    Posts: 8,057


    TONS of good advise here. The only thing I can add is the your hand is the worst sanding block ever. Always use a high quality sanding block.
  26. GKM
    Joined: Dec 20, 2014
    Posts: 105


    I always use the foam sanding boards, mine are about 12 in. Long depending on the surface I'm working.
  27. porknbeaner
    Joined: Sep 12, 2003
    Posts: 41,007


    Yea some body men make specific boards shaped to work a contour but you can also use a flexy board and trust the board and not your eyes.

    Something else that should be added, lay the bondo on in thin coats and press the spat really hard. No more then 1/8" per coat and pressing the spatula ( or whatever you call what you are using) will ensure less bubbles and less pitting in your finished product.
  28. What do you recommend?

    Charlie Stephens
  29. odass
    Joined: Mar 19, 2013
    Posts: 127

    from se ok

    I've been using rage gold with good success
  30. dcs13
    Joined: Sep 28, 2010
    Posts: 92


    Rage is what a lot of folks are using. I have tried their new Quantum and love it. It's tons easier than bondo to me.

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