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Thread: Rust proofing after cutting threads

  1. #1
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    Default Rust proofing after cutting threads

    I need to make up several very long bolts for my build (some as big as 40 inches). I found a good price on 5/8" galvanized bar stock that I can cut to length and thread to my specifications.

    After I cut the threads, and there by remove all the galvanizing from the bar stock in the threaded area, what is the best way to treat the exposed steel to prevent/avoid the threads rusting? Epoxy? Paint?

    appreciate the input.
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  2. #2
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    Default Re: Rust proofing after cutting threads

    I reckon I'd use either cold galv’ spray paint or a fish oil rust guard paint.
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  3. #3
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    Default Re: Rust proofing after cutting threads

    Cold galvanising paint.

    Is there not a galvanising tank near you? Make the bolts out of black steel, cut the threads with a big tolerance and send them for galvanising. You can fit the nuts that will become the head and peen or weld them to lock them on before galvanising.
    It really is quite difficult to build an ugly wooden boat.

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  4. #4
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    Default Re: Rust proofing after cutting threads

    NIck's advice is sound and what I would choose to do.
    One other way to go is cold galvanize. The product you would have the best results from is Devcon-Z cold galvanize. The product is 96% pure zinc.
    Jay

  5. #5
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    Default Re: Rust proofing after cutting threads

    Obtaining nuts for galvanized threads can be challenging. Make sure the ones you buy fit the rod properly before leaving the fastener place.

  6. #6
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    Default Re: Rust proofing after cutting threads

    Can you somehow finish them that they are completely buried in epoxy?

  7. #7
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    Default Re: Rust proofing after cutting threads

    Quote Originally Posted by robm View Post
    Obtaining nuts for galvanized threads can be challenging. Make sure the ones you buy fit the rod properly before leaving the fastener place.
    That is why you must cut the threads with a generous tolerance before galvanising. There will be a standard published somewhere.
    It really is quite difficult to build an ugly wooden boat.

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  8. #8
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    Default Re: Rust proofing after cutting threads

    What about inside of nuts? Isn't there some sort of goo that will go on threads when you screw together to stop corrosion?And cut so that there's only just the right amount of threads?
    ??????

  9. #9
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    Default Re: Rust proofing after cutting threads

    Quote Originally Posted by andrewpatrol View Post
    What about inside of nuts? Isn't there some sort of goo that will go on threads when you screw together to stop corrosion?And cut so that there's only just the right amount of threads?
    ??????
    Buy hot dip galvanised nuts.

    There should be one turn of thread under the nut and two turns out side of the nut, the first so that you can guarantee tightening down, the second for strength.
    It really is quite difficult to build an ugly wooden boat.

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  10. #10
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    Default Re: Rust proofing after cutting threads

    Would lanolin or similar be useful on the threads, even if hot dip galvanized? I suspect that crevice corrosion would eat the zinc off in a hurry, and then rust would bond the nut to bolt. Lanolin or whatever would fill the gap and prevent this. There would be a good size gap, as the nuts have to be loose threaded to fit the threads on the galvanized rod.

  11. #11
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    Default Re: Rust proofing after cutting threads

    I would buy plain bar stock, thread them myself and have them hot dipped.
    I have made quite a few machined and threaded parts for hot dipped galvanized and there are standard oversize taps (for female threads) and undersizes for male threads.
    You can purchase ready made taps and the better quality dies can be adjusted to cut undersizes. Off the top of my head 1/32" to allow for Hot dipped galvanizing.
    Here is a chart, but my own experience is a bit more than what they are advising is required.

    https://www.galvanizeit.org/images/u..._Fasteners.pdf

  12. #12
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    Default Re: Rust proofing after cutting threads

    Quote Originally Posted by robm View Post
    Would lanolin or similar be useful on the threads, even if hot dip galvanized? I suspect that crevice corrosion would eat the zinc off in a hurry, and then rust would bond the nut to bolt. Lanolin or whatever would fill the gap and prevent this. There would be a good size gap, as the nuts have to be loose threaded to fit the threads on the galvanized rod.
    Yes. Lanolin is good. One brand name to look for is Fluid Film.

    The thickness of a coating such as galvanizing or plating (zinc plating is useless) will change the pitch diameter by 4 times the coating thickness. So if the galvanize is .002 inch thick the pitch diameter changes by .008 or a lot more because it will tend to build up in the threads. That is why all of the advice about threading. The threads are cut at a 60 degree angle, the math works out to 4x.

    I would look for a good rust inhibiting epoxy primer and install the nuts on wet primer. Then apply the cold galvanizing after the primer dries. They aren't coming off easily, so this isn't for something that will need adjustment.

    Wizbang's advice is simpler, and good as long as you can live with encapsulated bolts.

    Edit: What are you bolting up?
    Last edited by MN Dave; 04-06-2017 at 12:59 PM.
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  13. #13
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    Default Re: Rust proofing after cutting threads

    Quote Originally Posted by MN Dave View Post

    I would look for a good rust inhibiting epoxy primer and install the nuts on wet primer.

    Why not simply use epoxy? Like West, System 3, or whatever one is using. A non-blushing variety might be more useful. After all, epoxy is often used as a paint primer… at least, a paint primer is not needed over epoxy. (in many people's opinion)

    Jeff

  14. #14
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    Default Re: Rust proofing after cutting threads

    Quote Originally Posted by jpatrick View Post
    Why not simply use epoxy? Like West, System 3, or whatever one is using. A non-blushing variety might be more useful. After all, epoxy is often used as a paint primer… at least, a paint primer is not needed over epoxy. (in many people's opinion)

    Jeff
    The reason for a rust inhibiting primer is that if there is a scratch that exposes bare metal as is likely on a thread tip, a plain epoxy will not prevent the corrosion from working its way under the coating. Installing the nuts on wet primer is an accepted navy and air force practice for installation to prevent corrosion. It gets away from the need for special taps and dies and galvanizing. Galvanizing is fine for low strength fasteners, but the heat weakens the high strength heat treated steel, so the aircraft can't use it.

    Epoxy is a barrier coating. It will prevent rust as long as it is thick enough and completely encapsulating the steel so that no water reaches the steel anywhere. Almost all coatings will pass some water vapor, but unless there is some salt trapped under the coating, that is not what I am talking about. There is only one paint that I know of that is essentially impervious to water vapor.

    Wooden boats don't need a lot of high strength fasteners because wood is too soft to utilize the strength. By the time you torque a 1/2-13 Grade 8 bolt to 75% of yield, the preload is over 14,000 pounds. When a Janka hardness of 1000 pounds is considered fairly hard, you can see where you won't be using the bolt's full potential. Even a galvanized threaded rod made from hot rolled A36 bar can be torqued to a 4600 lb preload, whic is still too much for anything without a really big, thick washer.

    If you want a rot and corrosion proof drift pin, use pultruded fiberglass. It does not work so well as a threaded fastener because threading cuts through the fibers. There are ways around that, involving epoxy bonded metal ends.
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