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Thread: Rusty mold help

  1. #1
    Boolit Bub
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    Question Rusty mold help

    I got an RCBS 38-150-KT mold in some trading that has surface rust in the cavities. I didn't see any pitting, but may have to spin polish them after I get them clean. What would be the best way to remove the rust?

  2. #2
    Boolit Buddy
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    Try EVAPO-RUST before doing anything mechanical. Environmentally friendly.

  3. #3
    Boolit Grand Master

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    You might try casting a few to see how they come out before doing anything. light rust may not hurt. If it does you have slugs to polish the cavities with. Here add a stem to the bullets and use tooth paste, simichrome or flitz for compound.

    But first clean the mould with dish soap and water with a tooth brush/ and cast some. then go from there

  4. #4
    Boolit Grand Master Bazoo's Avatar
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    I've used a bronze bore brush with brass center to clean cavities in a mould half. Secure the block half and spin the brush in a drill. I didn't notice any problems afterwards. Don't make a habit of it.

    Forgot to mention, I hold the end of the bore brushes center with my off hand to control it. I work at a slight angle so the end of the center is past the cavity end (bullet nose). I also try to have the brush spin so it sweeps away from the cavity edges, changing directions from one edge of the cavity to the other.

    I want to note for any new folks that might happen across this. I've only done this to a couple moulds that were old and rusted, not as a normal cleaning routine on good moulds.
    Last edited by Bazoo; 01-16-2021 at 03:35 PM.

  5. #5
    Boolit Master
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    If I trade for a rusty something I tape it off and use a bead blaster with 40 pounds of pressure with sugar sand. It removes no metal. I don't even use gloves. I did this in the carburetor repair business.

  6. #6
    Boolit Master
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    put it in some coke cola over night

  7. #7
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    Winger Ed.'s Avatar
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    Where ever that rust is, there is a pit under it from growing in the first place.

    Unless its bad enough to takes steps to fill the pits with pencil lead or something:
    With oil, heat, cleaning, and doing that several times, it might cure itself like a cast iron frying pan seasons itself with use.
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  8. #8
    Boolit Buddy
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    The metal of the mold is softer than you think it is,
    so start with the least aggressive method and go from there.

    I have had good results with soaking the mold face up in a light penetrating oil a day or two,
    then using a simple Q-tip in the cavities...you may be surprised how much surface rust comes off.

    Next is bronze wool, available at Home Despot, it's a lot softer than a bronze brush.

    Removing even a tiny bit of metal from the mold will increase the size of the boolit it casts.
    .
    .

  9. #9
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    Winger Ed.'s Avatar
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    Quote Originally Posted by Cord View Post
    then using a simple Q-tip in the cavities....
    Except for a tooth brush, that's all I've ever used, and then felt guilty sometimes if I dug around with them real hard.
    I was always a little gun shy to go after one with a bronze brush or wool.
    I've thought about using a green scotch-brite pad a couple times, but never did.

    After a couple go arounds with light rust, and having one mold trashed-
    I sort a got mad and started storing them submerged in motor oil.
    In school: We learn lessons, and are given tests.
    In life: We are given tests, and learn lessons.


    OK People. Enough of this idle chit-chat.
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    EVERYONE!
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  10. #10
    Boolit Buddy
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    Quote Originally Posted by Winger Ed. View Post
    ...and then felt guilty sometimes if I dug around with them real hard.
    That's a good point....The freed-up rust on the Q-tip acts like an abrasive,
    so it's a good idea to switch often to a clean Q-tip as soon as one gets dirty.

    After a couple cycles of soaking, a tiny wisp of bronze wool wrapped around the Q-tip
    is the next step for me.

    When done the cavities might look stained, maybe an orange-ish brown.
    If the Q-tip and oil picks up no more dirt and stays clean,
    its time to clean the oil off and try casting a few.
    .
    Last edited by Cord; 01-16-2021 at 09:27 AM.

  11. #11
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    100%. EVAPO-RUST!!

    Once ya try it you will never be without it!!


    It will Remove all bluing but a few casts and it will be "blue" again.

    Done this a few times. Wonderful Product.
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  12. #12
    Boolit Master
    Bent Ramrod's Avatar
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    If it’s just surface rust, I would boil the blocks in water for ten minutes or so. (Longer if you are living above 4000 ft altitude.)

    This converts the red rust (which tends to go on rusting) to black rust, which is stable, and basically the same stuff as the blued surface of the mould as new. You can watch this happening by taking a block half out, briefly, examining it, and putting it back into the boiling water. The orange color should fade to black in short order.

    Some of it (the larger bits) will blacken and fall off the mould, but the rest will set in place, like a rust bluing job, and protect the surface underneath. I then take the blocks out, let the moisture evaporate, wipe everything down with a paper towel, assemble, and cast a few for effect. If I’m not casting right away, I use WD-40 to protect the surfaces, and clean it off ahead of the casting session.

    Often, the black rust fills what otherwise be fine pitting, and the castings come out smooth. If there is slight pitting, the frosty look on the castings doesn’t seem to hurt the shooting qualities.

    And unlike lapping, I don’t have to break the mould in by rebluing the cavities by heating and casting, which is often necessary for the best boolits.

    Deeper rusting requires longer boiling, or multiple sessions of boiling, and will leave pitting, but the fine flash rust (IME) responds very well to this treatment. If it doesn’t work, more aggressive steps can be taken, as others have described.

  13. #13
    Boolit Bub
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    Thanks everyone for the advice. Before I got these molds I only had brass and aluminum. If I can save it, I'll be careful to avoid rust. The other two iron molds in the deal are perfect condition. But I don't have any short term plan to use them. Thinking about storing them coated with axle grease, wrapped in gun paper.

  14. #14
    Boolit Grand Master Bazoo's Avatar
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    I coat my iron moulds in mineral oil applied with an artists paintbrush. I remove it via lighter fluid. First I wipe the excess with a rag then spritz the mould with fluid, (make sure it's dry before heating). Then I heat it, open the cutter and fill each cavity with lighter fluid. It boils on the cavities and evaporates out. Man it cleans a cavity good! And it doesn't slow down the casting should you need to do it from contamination during a session. Make sure it all is gone and mould is dry before pouring into the cavity or putting the mould into the melt to heat.
    Last edited by Bazoo; 01-18-2021 at 05:10 PM.

  15. #15
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    mdi's Avatar
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    I have polished some cavities in my aluminum molds. I used Comet and oil paste on a cast bullet from the mold, mounted on a screw/stud in my hand drill. Works without removing much/no metal. Barkeeper's Friend is even less abrasive than Comet and that would be a good choice too...
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