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Applying Tar to Steel subframe to stop rust in low lying deck

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  1. #1
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    Default Applying Tar to Steel subframe to stop rust in low lying deck

    Hi, my deck is going to be built using a Steel C Section Subframe Galvanized Coldformed C100 as the subframe. My deck is quite close to the ground, I think the builder said he needs at least 30mm clear beneath the bottom of the frame, but would be happier if we can dig out 50-60mm.

    We are laying the actual timber decking over this ourselves. The builder suggested that we get some Tar from bunnings to coat the steel subframe as this will prevent any rust ever happening (because it is close to the ground).

    Just wondered if anyone has any experience doing this? What exactly should I tar - posts and bearers and joists? Top and bottom or just bottom (as top will have timber screwed into it)?

    Thanks

  2. #2
    4K Club Member ringtail's Avatar
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    All steel in the ground (or concrete) should be hot dipped gal. Normal gal will corrode. Putting "tar" on normal galvanised steel before it goes in the concrete has been the fashionable thing to do for years. It does next to nothing to protect the steel and in most cases can make corrosion worse. A better system is to use a 2 pack epoxy paint specifically designed for this application ( if the posts are not hot dipped). As far as what to coat, posts only would be sufficient but the more the merrier. You will never stop " any rust ever happening" but the best way to prevent it is to use hot dipped steel only, grade the ground under the deck so water runs off properly and make sure there is adequate ventilation under the deck.

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    Quote Originally Posted by SunnyDaze View Post
    Hi, my deck is going to be built using a Steel C Section Subframe Galvanized Coldformed C100 as the subframe. My deck is quite close to the ground, I think the builder said he needs at least 30mm clear beneath the bottom of the frame, but would be happier if we can dig out 50-60mm.

    We are laying the actual timber decking over this ourselves. The builder suggested that we get some Tar from bunnings to coat the steel subframe as this will prevent any rust ever happening (because it is close to the ground).

    Just wondered if anyone has any experience doing this? What exactly should I tar - posts and bearers and joists? Top and bottom or just bottom (as top will have timber screwed into it)?

    Thanks
    because your not putting the steel in concrete it could only be a plus to paint it with a bitumen coating, this method is a recommended corrosion resisting treatment for gal steel in the masonry standard in corrosive environments.
    regards inter

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    4K Club Member ringtail's Avatar
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    Who says it's not getting concreted ?

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    Quote Originally Posted by ringtail View Post
    Who says it's not getting concreted ?
    so far your the only one who has mentioned steel in concrete, when my understanding is the steel is "quite close to the ground"
    Regards inter

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    Community Moderator phild01's Avatar
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    huh...

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    Thanks guys. I am assuming the posts will be concreted into the ground, but the frame will be close to the ground. The specs say GALVINISED posts - hot dipped is not specified. But I will clarify this with the builder.

    So if it is hot dipped galvanised, I don't need to do anything? But if it is just galvanised I need to apply the epoxy. Have I understood correctly?

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    Community Moderator phild01's Avatar
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    Normally your posts are made to spec and sent off for hot dip zinc. Otherwise, Duragal steel may be used. Thing with Duragal is that the inside of RHS can be unplated and will rust, whereas the imported lookalike is done inside and out and preferable. If any cold zinc spray is used, it should not be in the ground. In your instance I would prefer the steel fabrication and hot dip, though more cost.

  9. #9
    1K Club Member Random Username's Avatar
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    Just to straighten the nomenclature out:

    Galvanised = hot dip galvanised = dipped into a bath of molten zinc. Has that unmistakeable 'flake' style pattern. Gal screws tend to have a slightly rougher feel than other screws. Can be purchased off the shelf, or custom sections can be sent away for dipping. (note that considerations apply for dipping, so use an experienced fabricator if custom making)
    Zinc plated = silvery finish achieved by electroplating zinc onto steel. Nowhere near as thick or durable as gal. Decorative inside-use only process, not for exposure to the real world. Sometimes it is passivated (extra process) to give it a golden-ish tint, but still nowhere near as durable as gal. Zinc or zinc passivate is what you'll find on typical small bright steel nuts and bolts on your car.
    Duragal = hot dip galvanised with an extra layer of a (proprietary???) surface coating to reduce the zinc's white rust tendency.

    I work on the theory that the smoother and nicer a zinc coating looks, the less effective it is!

    Here's what I'd do:

    Once the steel has 40mm or more of good concrete around it, there's so little oxygen available that rusting isn't really an option; BUT zinc is a reactive metal, so a coat of bitumen can't hurt, especially where the steel exits the concrete.

    Most likely spot for rust will be at any bolts (tightening them up can scratch through the gal), so if you are really keen, apply some cheap Vaseline to the threads/washers before screwing up, wipe off excess after tightening and then paint over with bitumen. Then paint over again a few days later just for luck. You could spray on some of those thick fish oil compounds instead of the Vaseline if you prefer. Also apply a coat or two of bitumen between any metal sections that touch before screwing together.
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  10. #10
    Community Moderator phild01's Avatar
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    Quote Originally Posted by Random Username View Post
    Just to straighten the nomenclature out:

    Galvanised = hot dip galvanised = dipped into a bath of molten zinc. Has that unmistakeable 'flake' style pattern.
    Can't agree here.

    I am not sure why you say hot dipped into molten zinc has the 'flake' look. What I have ever seen is a smooth consistent zinc look, no flakes. The flake look that is so evident on galvanised downpipe, as opposed to the stipple look on zincalume downpipe, is not a hot dipped product. It is an applied zinc flake system that work on various different principles.

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    Recently I was involved in adding 17t of concrete on top of the existing base of a comms tower. The tower manufactures recommended the following "Tower members embedded in concrete and 75mm above the surface to be coated with Dulux Durebuild"

    The tower members were hot dipped galvanized steel

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    1K Club Member Random Username's Avatar
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    Quote Originally Posted by phild01 View Post
    Can't agree here.

    I am not sure why you say hot dipped into molten zinc has the 'flake' look. What I have ever seen is a smooth consistent zinc look, no flakes. The flake look that is so evident on galvanised downpipe, as opposed to the stipple look on zincalume downpipe, is not a hot dipped product. It is an applied zinc flake system that work on various different principles.
    I'm guessing you're thinking 'flake' as in little sparkly bits of aluminium in car paint or micaceaous iron oxide particles in industrial paints.

    I'm thinking 'flake' as in the boundary patterns formed by the molten zinc as it cools on the steel surface, usually noticeable from a meter or so away on street structures like lamposts, crash rails and handrails.


    Both galvanising and zincalume coating are hot-dip processes - both involve the base steel being drawn through a bath of either zinc (for galvanised steel) or zinc-aluminium alloy (for zincalume steel). In the manufacture of flat sheet, the coating (zinc or zincalume) thickness is controlled by a blast of air across the sheet as it comes out of the molten metal, which can impart a much smoother surface finish than the more typical dunk & drain approach used by most industrial galvanisers.

    I haven't seen non-BHP/Bluescope/Lysaght/Onesteel/whatever the heck they are called at the moment products with zincalume coatings - just the BHP sheet product. Everything else comes with standard galvanising. Zincalume is (supposedly) four times more corrosion resistant than standard gal, BUT it is not compatible with standard galvanising (makes it corrode faster).
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    Quote Originally Posted by intertd6 View Post
    because your not putting the steel in concrete it could only be a plus to paint it with a bitumen coating, this method is a recommended corrosion resisting treatment for gal steel in the masonry standard in corrosive environments.
    regards inter
    Scratch that about bitumen coatings in the standard I had a look & couldn't find that reference. Going back through the fog of memory I had a large project near the beach (less than 500m) that specified the steel lintels to be hot dip galvanised & bitumen coated.
    regards inter

  14. #14
    4K Club Member ringtail's Avatar
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    Quote Originally Posted by SunnyDaze View Post
    Thanks guys. I am assuming the posts will be concreted into the ground, but the frame will be close to the ground. The specs say GALVINISED posts - hot dipped is not specified. But I will clarify this with the builder.

    So if it is hot dipped galvanised, I don't need to do anything? But if it is just galvanised I need to apply the epoxy. Have I understood correctly?
    Yes, correct. The product that Bros mentions or similar would be the one to use to coat anything regardless of whether its in the concrete or close to the ground. Coating the hot dipped posts with it certainly wont hurt at all but is a bit of overkill IMHO. However, if you were to buy some to do the other bits you may as well go for best practice. Most important though, make sure the posts are hot dipped and dont take any BS from the builder about it.

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