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Rust Prevention - Cold Gal(v)

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A search of the forums only turns up one reference to using Cold Gal(v) as a rust prevention coating whilst there are more mentions of using POR15 and KBS rust seal paint treatments. Not sure of the principal behind the POR15 and KBS paints however given the proven characteristics of galvanising I suspect the cold gal(v) type paints with >90% zinc content would be the better option especially when painted onto the  bare metal as an undercoat either on internals of the doors/hatch, the floor pans and/or on the underbody.

The photo below shows how a fence/gate post in a coastal location  (~100m from the beach) has survived 50 years of salt exposure with a coat of cold gal(v) whilst the barbed wire has not fared well!

Anyone used Cold Gal on their Zed?

 

IMG_8196.thumb.JPG.73090d7ccab5d597113085c6dd0d8a6f.JPG

 

 

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That gate and gate post has clearly been hot dipped galvanized during manufacture, where the zinc coating is bonded to the metal in a galvanizing plant's heated dipping bath. It will be inside the tubes as well, anywhere where the molten zinc could flow and reach. It is not a paint coating.

Zinc dissolves with rainwater very very slowly over time. Look at corrugated galvanized iron (cgi) roofing once used on old buildings before Zincalume and Colorbond became available. Over time the zinc literally washes off and the roofing starts to go rusty. And so the roof either gets replaced or painted to make it last a bit longer. In that picture above, the post is still fairly sound although pitted but the eyelets have lost their coating and are rusting. The barbed wire has lost its very much thinner zinc coating long ago and is now badly rusted.

'Cold Galv' is a zinc rich paint typically used in the construction industry to touch up areas - brush or rattle can - where hot dipped galvanized steel items have then had to have further work done exposing some bare steel again, such as welding on-site. Often used on long lengths of handrails for example. But a paint coating even in multiple layers cannot match the longevity of a hot dipped galvanized coating so the make-good work will always be a maintenance item.

It's not a good idea to paint over a zinc coating because even through the paint it can react with moisture to form the white salt powder you can see in places in your photo, and any paint coating detaches. Obviously zinc coatings used in inland dry areas will last far longer than in areas where there is moisture such as near the coast, areas of humidity, or where there's rainfall, areas where the majority of us live.

It has its place but I wouldn't use Cold Galv except in hidden areas not requiring a visible top coat, like inside doors/hatch as you suggest. And I'd coat it in turn with a rust inhibitor such as Fisholene or Tectyl or whatever. As long as it's applied to well cleaned dry steel and is itself protected it should last well. For painted areas exposed to view use a good quality automotive paint system which includes a high quality metal etch adhesion primer.

Apologies for the lecture! My 2c worth.

 

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From my (fading) memory, this gate never had the shiny silver hot dip gal "crystaline" finish, it was always a dull grey colour.

One has to ask, given the benefits of zinc for rust prevention, how do the alternatives POR15 and KBS rust seal achieve their "rust protection" properties - is it just through "sealing" out oxygen/moisture from contacting the metal?

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Posted (edited)

Yeah, that's not cold gal paint on the post. I used the gal paint on some handrails made from gal pipe I welded up for my old man's place, just around the welds. After 5-6 years the cold gal was starting to show a little rust coming thru (and yes, I gave it a few coats).

In my experience, your best bet is a good epoxy primer on visible painted surfaces (or POR15 / KBS if you're keen, I've used them on both car and trailer with reaonablt succes, admittedly the car hasn't left the shed since.....), and a good dose of something like fish oil / cavity wax in all cavities. My personal "special mix" used to be a combination of Tectyl rustproofing and a bit of fish oil to  thin it down and get it penetrating into all nooks and crannies.

Edited by 1600dave

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Posted (edited)
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From my (fading) memory, this gate never had the shiny silver hot dip gal "crystaline" finish, it was always a dull grey colour.

Hot dipped galv usually starts off fairly smooth and shiny - 'crystaline' as you aptly describe it - straight out of the bath but soon oxidizes and dulls down in the atmosphere. And depending on variables in quality the colour can vary and be patchy much to architects' disgust. Your dull grey gate and post was probably from older stock before it was finally installed.

There are also electroplated zinc coatings used for thin steel sections, such as panels eg. a/c ducts. It's a thin more economical coating really only suitable for interior work. It too has a bright shiny 'crystaline' finish, in fact the term is 'spangled' (lovely word!). But again over time the zinc ever so slowly dulls down even in an internal atmosphere.

Contrary to what people might reasonably assume, a weathered matt galvanized surface although it looks suitable for paint to mechanically stick to is not the best surface to paint over. In fact, it best needs to be fresh unoxidized zinc, so an aged coating should be taken back to clean zinc - 'sweep blasted' - before applying a suitable primer within hours not weeks or months, that's if one wants to gain maximum durability.

Ditto what 1600Dave says. Paint technology has moved on.

Edited by gilltech

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