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Thread: Block sanding high build primer

  1. #16
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    Steve, you didnt read the whole post. I blocked with 500 grit and sealed and some scratches re-appeared because I sprayed high build primer over 36-120 grit scratches. I created a problem for myself due to inexperience. I dont want to re-strip the whole car. There must be a way out of this.

    When block sanding i exposed both bare metal and filler, the filler shows in in the sealer.

    My jobber told me to reduce the expoxy 2:1:1. is this not correct?
    Is that picture you posted where you are now?What primer is there?Either way,i would block it with 180 then reprime it all.You can't prime one time over anything courser than 180 and expect it not to show scratches.
    What i do,is get my metal level as possible...apply filler,use 40 or 80 to level it out and once i get to the point where i know one more coat of filler will be it,i make the final coat polyester putty ,guide coat and sand that with 120,then guide coat again and finish with 180.Prime..several hours later sand with 180,and reprime.

  2. #17
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    Quote Originally Posted by Microgiant
    Steve, you didnt read the whole post. I blocked with 500 grit and sealed and some scratches re-appeared because I sprayed high build primer over 36-120 grit scratches. I created a problem for myself due to inexperience. I dont want to re-strip the whole car. There must be a way out of this.

    When block sanding i exposed both bare metal and filler, the filler shows in in the sealer.

    My jobber told me to reduce the expoxy 2:1:1. is this not correct?
    No, I think I did understand. You have primer final blocked to 500 then sprayed with sealer and you have scratches showing in your sealer. Is this not where you're at? If so, sand everything with 220 (so that another coat of high build has something to hold on to). This will serve to fill the scratches left behind. If the only problem you had at the point of sealer was the visible scratches then just sand this last coat of high build with 400, or 500 or 600. If the high build is a little rough and peely I would cut it first with 320 or 400 to knock down the roughness then polish it up with the 600.

    Yes, your jobber is wrong. Always have the tech sheets for the products you're using on hand. 170 is 2:1 reduced 10%RTS with acetone.

    BTW, sealer should be sprayed immediately before basing, as if it were your first coat of base.

  3. #18
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    I think that that particular scratch re-apeared due to primer shrinkage.

    Also this thread says to reduce MP170 2:1:1. I have to admit with that much reducer it had the consistency of water.

  4. #19
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    Quote Originally Posted by Microgiant
    I think that that particular scratch re-apeared due to primer shrinkage.

    Also this thread says to reduce MP170 2:1:1. I have to admit with that much reducer it had the consistency of water.
    Why the scratch appeared is irrelevent at this point. It has finished shrinking and now needs to be filled. There's no need to worry about removing the root cause of the scratch.

    In that thread I stated that 170 was mixed 2:1 with 10% acetone optional. That is what the manufacurer states on their sheet. Go to PPG's site and print off the sheet for yourself. I'm not one to play amateur chemist and belief that if that's what the manufacuter says that's what I'll do. Sprayed at 10% reduction it lays out beautifully, nice and flat, which is the primary reason for the additional reduction for use as a sealer. I don't know why anyone would want to reduce it anymore, expecially if it goes beyond the manufacurer's recommendation.
    Steve g

  5. #20
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    Thanks for your help Steve. Im going to give this a try.

  6. #21
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    I think everything is going to be alright!




    Its getting a vinyl top btw.

  7. #22
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    Quote Originally Posted by Bob K
    I am not answering your question but just making a comment on your choice of priming over 80 grit scratches. I have done it and I have seen the print of the scratches show up later in the final paint job. I now only prime over 180 or finer scratch. This may work out for you if you let enough time pass so all the shrinking of your surfacer is done before you sand. Others may side with you on surfacer over 80 but I do not like to do it.

    Bob K
    I COMPLETELY agree with this comment. I have personally experimented with sand scratches and primer. regardless of the solids content of the primer (MS vs HS/High Build), if you do not let it fully cure for atleast 4-6 hours, letting the primer naturally shrink as it is designed to do, the scratches will APPEAR to be gone until you apply color, and sometimes, you wont even see them until it is cleared. This will set you back quite a bit. 180 is THE coarsest that i prefer to prime over, and depending on the color (esp. blacks) i will take it as far as 220 before primer, to assure proper filling, hiding, and coverage.

  8. #23
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    Quote Originally Posted by Chris L
    ...i still get shrinkage in primer and thats finishing with 180 before prime.I have use K38,matrix,dynatone etc..all of those 2k primers shrink..So far the only way for me to not have sand scratches showing is if i prime,block with 180 3-4 hours later and re prime THEN the next day block with 400 and up and paint.
    Matrix's MP4-2K Lightening Primer is designed to shrink and cure at 77 degrees F in about 1-1 1/2 hours. there is no need to wait NOR reprime the panel. This creates more work and you have now applied more product then necessary.

    HINT:::

    LESS IS ALWAYS BETTER!!!

    this not only applies to the person paying for the materials, but also to the person who is spraying the product and who's name is on that job.

  9. #24
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    Matrix's MP4-2K Lightening Primer is designed to shrink and cure at 77 degrees F in about 1-1 1/2 hours. there is no need to wait NOR reprime the panel. This creates more work and you have now applied more product then necessary.

    HINT:::

    LESS IS ALWAYS BETTER!!!

    this not only applies to the person paying for the materials, but also to the person who is spraying the product and who's name is on that job.
    Thanks,I'll try some of that next time since I use Matrix.Their mp3?i forget the number ,had the same typical results.How many coats of mp4 are you doing?

  10. #25
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    if you are using the primer as a SURFACER, try to spray no more then 2 medium-wet coats. If you are using it as a SEALER, reduce it with a 0800 series reducer. Mix it 4:1:10%, and only spray out one full wet coat. Something else that i've found with the the MP4-2K...you want to get the panel temp to about 77 degrees F, give or take a few degrees. Once you are finished spraying, let the panel COOL...if you are using a heater to heat the booth, kick it down to about 65. It seems to almost SHOCK the primer and it will actually dry quicker. Same goes with the MH43 Panel Hardner for clear.

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