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  1. #1
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    Default Prepping OLD cast iron for finishing?

    Hi all -

    New to the forum. Have a general question, with zero experience. I'm restoring a ca. 1920s wine/fruit press:
    IMG_2592a.jpg

    The entire thing is cast iron. And completely rust free. I know almost nothing about cast iron, so don't know if this is coated with something, or somehow passivated, but it's remarkably clean. However, iron contamination in wine, which was a thing back then (and is largely not now), is a real possibility if this press is used without somehow coating the iron before use. I'm looking into powder coating the base, which would involve some sort of abrasive blasting first, or cleaning up the metal and painting with a food safe enamel. I'm not sure, though, what the best way to do that is.

    Is there a way to determine what, if anything, is on the surface of this iron? Does it need to be prepped at all before painting, should I go that way? What's the best way to do that? Is there a finishing schedule I should follow?

    Thanks for any direction you may give. This is an over-winter project, so I'm not hurrying into anything, but want to do this correctly.

    Thanks,
    Scott

  2. #2
    Join Date
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    Default

    Hi Scott,

    Welcome to to the MetalWork forums'

    To help you explore the other areas of our forum here's a guide on to to navigate our Forum and find all the interesting places.

    Goto the FORUM box in the top left hand cnr of the page and click the down arrow. This will bring up a pull down menu that has Forum Home at the top

    Click Forum Home which will present a scroll down page.

    Our rules,the Terms of Service are right at the top ,we encourage you to read them.

    Below that are all the various areas and sub forums that make up out MetalWork forums.


    Welcome

    Grahame

  3. #3
    Join Date
    Aug 2015
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    Melbourne, Australia
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    Default

    Hi Scott, welcome to the forum.

    Not that I know much about these thing but might it be the same sort of thing as done to season cast iron cookware? Like, effectively bake/burn on oil.

    I know that my camp oven (which is a cast iron pot with a heavy lid of same) needed it. Otherwise it rusts. The layer of burnt oil on it seals it.

    Greg.

    EDIT: and great thing you're doing a resto. Share some pics.

  4. #4
    Join Date
    Sep 2012
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    York, North Yorkshire UK
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    Default

    Hi Scott, Guys,

    Welcome to the forums Scott.

    The coating that is on the cast iron is caused by the tannin in the fruit ! As Greg suggests its the same sort of thing as seasoning on cast iron cookware. You will find that if the do try to remove it you will need to remove a lot of the surface metal as well. I would simply wash it before use to remove any debris that remains.
    Best Regards:
    Baron J.

  5. #5
    Join Date
    Oct 2020
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    Default

    Thanks for the welcome guys!

    Quote Originally Posted by BaronJ View Post
    The coating that is on the cast iron is caused by the tannin in the fruit ! As Greg suggests its the same sort of thing as seasoning on cast iron cookware. You will find that if the do try to remove it you will need to remove a lot of the surface metal as well. I would simply wash it before use to remove any debris that remains.
    Fascinating! And very good news - the cost of this project just dropped significantly. I've done some reading now on the formation of iron tannate and, considering the age and condition of this press, I'd guess that any available surface iron has been chelated. And, from what I've read, the tannate is pretty much insoluble and inert. So I feel pretty good about using this without any further treatment of the iron, expecting minimal, if any, leaching of iron into the must. Thanks so much for pointing this out!

    Quote Originally Posted by StrayAlien View Post
    EDIT: and great thing you're doing a resto. Share some pics.
    Will do. I'm looking forward to putting this thing to work in the spring Chilean harvest. The basket is getting rebuilt currently (QS white oak, maybe fumed, and stainless bolts). I think the head may get a coating of Boeshield, but otherwise the iron will stay as-is. Pictures of the current state can be seen at https://flic.kr/s/aHsmRBUEdE.

    Thanks again!
    Scott

  6. #6
    BobL is offline Member: Blue and white apron brigade
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    Feb 2006
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    Perth
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    Default

    I would wash it and then press some grapes in a couple of small batches before doing bigger batches. If there is any loose stuff it should come away in the first batch or two and it should convert any newmicro rust spots

    I've done quite a few experiments with Iron tannate on rusted surfaces.
    I bought tannic acid powder and made up a solution of tannic acid according to a recipe developed by a Canadian Museum.
    My experience is that while it is indeed relatively insoluble is it not that physically robust so if it is constantly rubbed by another hard or abrasive surface it will come away and expose fresh metal. I would make sure that there is not any significant rubbing anywhere on the press that is going to be soaked in juice.

    If you find any rust spots on the press, the tannate protection seems to work best if the rust is left on and tannic acid is applied to the rust. The tannic acid converts the rust to tannate which seems to have better adhesion that when applied to bare metal. if any rust is loose then just brush it off with a wire brush.

    Here is a blacksmiths vice that underneath that blue paint was rusty as.
    The paint was removed with a wire wheel on an angle grinder but the firm rust was largely left alone and converted to tannate using multiple (>5) coats of tannic acid.
    The shine is a from a coat of boiled linseed oil and then left in the sun for 3 weeks to polymerise into a hardish overcoat.
    I would not recommend using the linseed on your press.
    FinalBSvice.jpg

  7. #7
    Join Date
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    Default

    Quote Originally Posted by BobL View Post
    I would wash it and then press some grapes in a couple of small batches before doing bigger batches. If there is any loose stuff it should come away in the first batch or two and it should convert any newmicro rust spots

    I've done quite a few experiments with Iron tannate on rusted surfaces.
    I bought tannic acid powder and made up a solution of tannic acid according to a recipe developed by a Canadian Museum.
    My experience is that while it is indeed relatively insoluble is it not that physically robust so if it is constantly rubbed by another hard or abrasive surface it will come away and expose fresh metal. I would make sure that there is not any significant rubbing anywhere on the press that is going to be soaked in juice.

    If you find any rust spots on the press, the tannate protection seems to work best if the rust is left on and tannic acid is applied to the rust. The tannic acid converts the rust to tannate which seems to have better adhesion that when applied to bare metal. if any rust is loose then just brush it off with a wire brush.

    Here is a blacksmiths vice that underneath that blue paint was rusty as.
    The paint was removed with a wire wheel on an angle grinder but the firm rust was largely left alone and converted to tannate using multiple (>5) coats of tannic acid.
    The shine is a from a coat of boiled linseed oil and then left in the sun for 3 weeks to polymerise into a hardish overcoat.
    I would not recommend using the linseed on your press.
    FinalBSvice.jpg
    Thanks @BobL for the great description. Good information to have. There isn't a lot of movement at the bottom of the press in use, other than the basket just sitting on the pan of the base. So I'm not sure if there will be an issue in practice. I kind of like the idea that, with use, it's kind of a self-renewing finish. It needs a good cleaning right now but it looks to be in good shape.

    A couple questions for you regarding resilience: First, do you think pressure washing is appropriate, or is that going to degrade the coating to quickly? I have a small electric pressure washer (Karcher) that's about good for washing cars and not a lot else. Otherwise I'd probably just sponge it down with PBW (a percarbonate cleaner, similar to OxiClean) and rinse with a hose. Second, a very common sanitizer, and the one I use, is StarSan (https://www.piwine.com/media/Product...DS-STARSAN.pdf). It's an acid-based sanitizer, largely phosphoric acid. I think this would be okay in use with the tannate coating, but have read that the formation of tannate can be reversed with varying pH. Since tannic acid is, of course, acidic, I'd think this would be good. Have you any thoughts on this?

    Thanks again for sharing your experience. The vise is a beautiful piece of equipment!

    Scott

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