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Thread: 2K High Build Primer / Block Sanding

  1. #1

    Default 2K High Build Primer / Block Sanding

    I have a question on sanding 2k high build primer. I have sprayed 2 coats of high build primer and have blocked it out with 180 grit. I plan on blocking again with 220 then 320 grit. Do I need to respray between grits?

  2. #2

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    Only if you sand through.
    I can't imagine not breaking through with 180 grit and only 2 coats of primer.
    I usually sand with 150 to 180 then guide coat and sand with 400 to 600
    to get rid of any scratches, then it's ready for paint.
    But I put way more primer on and I still break through once in a while.

  3. #3

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    Can you use a DA instead of a long block on a hood?

  4. #4
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    Quote Originally Posted by GasBuggy View Post
    Can you use a DA instead of a long block on a hood?
    There is a wide gray area answering that question. Could a pro using a correct grit paper DA sand (not block sand) a hood ? Yes.

    Should a novice use a DA to block sand a hood ? Absolutely not. Novices should always block sand primer by hand.

    A novice trying to block sand anything with a DA sander will dig divits into the the paint/primer with the end result of the surface looking like an old washboard (uneven surface).

  5. #5

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    Quote Originally Posted by JCCLARK View Post
    Only if you sand through.
    I can't imagine not breaking through with 180 grit and only 2 coats of primer.
    I usually sand with 150 to 180 then guide coat and sand with 400 to 600
    to get rid of any scratches, then it's ready for paint.
    But I put way more primer on and I still break through once in a while.
    I did one light coat then one heavy coat. Using 90% of the gallon of primer. The only spots showing right now are on some of the edges and a few that are smaller than a dime. It's very possible when I do the 220 I will break through in more spots. Hopefully not.

  6. #6
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    Quote Originally Posted by Malibu1966 View Post
    I did one light coat then one heavy coat. Using 90% of the gallon of primer. The only spots showing right now are on some of the edges and a few that are smaller than a dime. It's very possible when I do the 220 I will break through in more spots. Hopefully not.
    Plan on it. You can't go through two more grits without having some more burn through. Id do the 220 and then spot prime the burn through areas. Hit those with 220 and move to 400.
    b marler

  7. #7
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    Default Hmmmmmmm...

    Quote Originally Posted by Malibu1966 View Post
    I did one light coat then one heavy coat. Using 90% of the gallon of primer. The only spots showing right now are on some of the edges and a few that are smaller than a dime. It's very possible when I do the 220 I will break through in more spots. Hopefully not.
    Just curious, please. What did you paint that took 90% of a gallon of primer and was that before or after reducing it? Thanks.

    Henry

  8. #8

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    Hi Henry.
    I painted every thing except the fenders, hood and firewall. Mixing at a 4:1 ratio.

  9. #9

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    That's a lot of primer.

  10. #10
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    That is a lot of primer for certain. It was mentioned before but in my experience you can't overlook the guide coat (darker or lighter primer to reveal high/low spots) if you want a smooth straight finish. Possibly (if you're really good and careful) you could use a jitterbug sander but not a DA. Personally I'm not a fan of DA sanders for a couple of reasons, but that's another discussion.

    You might try one of the hand sanding tools made for drywall work. They're longer, have a fairly hard rubber pad, are wider and can be used wet or dry. They'll cover a lot more ground a lot quicker, they're inexpensive ($6 to $10) and for guide coat work they're incredibly better than the typical hand pad sanders. Good luck!

  11. #11

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    Quote Originally Posted by JonG View Post
    That is a lot of primer for certain. It was mentioned before but in my experience you can't overlook the guide coat (darker or lighter primer to reveal high/low spots) if you want a smooth straight finish. Possibly (if you're really good and careful) you could use a jitterbug sander but not a DA. Personally I'm not a fan of DA sanders for a couple of reasons, but that's another discussion.

    You might try one of the hand sanding tools made for drywall work. They're longer, have a fairly hard rubber pad, are wider and can be used wet or dry. They'll cover a lot more ground a lot quicker, they're inexpensive ($6 to $10) and for guide coat work they're incredibly better than the typical hand pad sanders. Good luck!

    Being new to painting, and reading that people put down 2 or 3 coats of high build prime. I put down 2 coats. But obviously way to heavy.

    Should I have done a first coat of High Build then sand with 180. Then another then sand with 220. Then another final and coat before sanding with 320?

    The colour that I will be painting is a Madeira Maroon. There are a few people on the forum that say they use 400 grit to finish. This is not a show car, so I am thinking that 320 should be ok with not showing the sand scratches. Maybe I'm wrong?

    The only time that I used my DA was to give my body filler a quick knock down, then block it out. I use the Durablocks for my block sanding and some that I have made for smaller sanding spots. For my guide coat I started using the black spray paint, then eventually switched to the 3M guide coat. I find it's much nicer to work with than the spray paint.

  12. #12
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    Quote Originally Posted by Malibu1966 View Post
    Being new to painting, and reading that people put down 2 or 3 coats of high build prime. I put down 2 coats. But obviously way to heavy.

    Should I have done a first coat of High Build then sand with 180. Then another then sand with 220. Then another final and coat before sanding with 320?

    The colour that I will be painting is a Madeira Maroon. There are a few people on the forum that say they use 400 grit to finish. This is not a show car, so I am thinking that 320 should be ok with not showing the sand scratches. Maybe I'm wrong?

    The only time that I used my DA was to give my body filler a quick knock down, then block it out. I use the Durablocks for my block sanding and some that I have made for smaller sanding spots. For my guide coat I started using the black spray paint, then eventually switched to the 3M guide coat. I find it's much nicer to work with than the spray paint.
    i usually do primer one coat at a time. first block with 180/220 should work fine to get things level, then work up the grits. second coat (if needed) i finish with 400 for solid colors, 600 for flake. 320 isn't generally fine enough, especially for dark colors. most times, only one coat is needed, and then spot prime any burn throughs, hit with 220,320 (maybe) then 400. sometimes i go right to 400 from 220, depends on if the 400 can remove the 220 scratch easily. sometimes the 400 is too big of a jump. different primers sand differently.
    b marler

  13. #13

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    Couldn't you use a da to knock the primer down some for the initial sanding? Save your arms some?

    Also, sorry to thread hijack, but do you need anything larger than a 12" block for the average car?

  14. #14
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    Quote Originally Posted by GasBuggy View Post
    Couldn't you use a da to knock the primer down some for the initial sanding? Save your arms some?

    Also, sorry to thread hijack, but do you need anything larger than a 12" block for the average car?
    Sanding filler primer using a DA sander can depend on a couple of variables. Since sanding the primer is usually the last step before spraying on your top coat the sanding needs to be fairly level especially under darker colors. Getting the surface perfect takes block sanding with the proper block and sandpaper but if the surface is going to be painted white or another light color AND it's a fairly flat surface you may be able to get away with using a DA sander.

    We have a 55 Buick station wagon in the shop and we had to strip the roof and we primed it with Optex primer and since it's high and white we may sand it with a DA but we haven't made that decision yet.

    If you want the panel perfectly level then block sanding it is the way to go but some jobs don't demand a perfectly level surface. In most caes we use Dura-Blocks from the large kit.


  15. #15

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    Thank you everyone for all your advice. Learning never stops and making mistakes along the way is just part of the learning curve.
    Malibu 1966.jpg

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