Painting galvanised steel

[PaulT]

I shall be finishing with smooth Hammerite - would you do any surface treatment / primer or just paint on the Hammerite?

[Chancer]

I hope that its not new galvanised, in order for paint to stick it must be left to weather for 5 to 10 years.

Chlorinated rubber paint sticks better than most, Hammerite smooth is probably the worse choice, the best is to use a bitumen tanking compound which has the advantage of coming in any colour you like as long as its black.

I had my trailer hot zinc sprayed, not wanting to wait for it to weather I painted it with Rubson bitumen compound, it took a summer and winter to not be sticky to the touch but 9 years on there has been no flaking.

[ventodue]

As Chancer says, it really depends on how old the surface is, Paul.  2 years +,  and the galvanised has started to acquire a surface layer of zinc carbonate which is well bonded to the underlying layer.  In this state, there's no surface prep to do, besides a clean to remove dust and muck.

Partially exposed galvanised can be a pain, however. The surface compounds (zinc oxide and zinc hydroxide) are relatively weakly attached electrostatically, and should ideally be removed before painting.  They can be washed off; but even so, if you've got a large area to paint, that can be a right PITA.  And even then, the result can be disappointing - DAMHIK.

Guess it all depends on what you're painting, how big a surface area, and what you want as a result ( I rather like Chancer's idea of bitumen paint ... [:)])

HTH

Craig

[PaulT]

Thanks for the replies.

It is a door of an outbuilding. We have owned the house for 8 years and it was not in the best painted condition then. Some of the existing paint has come off.

Think bitumen would be a problem - I can see me getting stuck to it :)

[Chancer]

Then you have the worst of problems, where the paint has flaked off and the galvanising exposed its now a good base for paint but you will have to remove all the rest of the paint and leave it a good few years before repainting during which time it will look patchy and odd.

I have a galvanised porte de service fitted to a store-room on the end of my flat, it faces out onto my patio and is the very last unfinished item, everything else looks great but its still only 9 years since I fitted it so I wont paint it until summer 2016 and even then only the outside, the Inside will probably never get done.

[crack]

I found  a little how to do it thing, its in french but use the translater

Peindre le zinc.

Le gris des gouttières en zinc n’est pas toujours très esthétique sur

une façade. On peut peindre le zinc avec la technique suivante :

Dégraisser le zinc à l’eau chaude additionnée de 100 g de lessive Saint-Marc par litre, ou au trichloréthylène.

Après séchage, appliquer une sous-couche spéciale pour zinc : un wash primer ou une sous-couche au chromate de zinc.

Appliquer ensuite deux couches de peinture décorative, soit une laque brillante, soit de la peinture de façade.

Peindre un jour sec, sans vent et sans un soleil trop violent.

[Théière]

Although that is in French it seems to make sense. It was back 20+ years ago but we used to clean off the surface and etch it with acid prior to painting and certainly the contaminants (oxide and hydroxide). Caustic will also etch the surface.

[ventodue]

[quote user="crack"] <snip>  ...ou une sous-couche au chromate de zinc.[/quote]

Zinc chromate primer!  Blimey, that brings back memories (as per Théière) ...[:-))]

Still ... worth a try, I s'pose [:)].

[Mr Wiggy]

In the days of yore when painting plated metals and aluminium the base coat that was used in the motor trade then was an etching primer which was a cyanide based primer. This was banned in the early 2000s with the introduction of water based paints. BUT I purchased an aerosol can in one of the super stores in Bressuire in 2006.

[Chancer]

All the etch primers for aluminium that I have used were acid based, they are still available.

In my experience, and believe me I have tried all the shortcuts paint will not adhere (properly, correctly, durably pick your word) to galvanised steel whatever the preparation and base coat until it has been exposed to the elements for many years, I say 10 years others have different times

[Chancer]

I see that Hammerite make this:

http://www.hammerite.co.uk/guide/direct_to_galvanised_metal_paint.jsp

According to them you wait 6 months, well I think they have just seized a marketting opportunity and like all snake oïl dressed up as the multitude of specialist problem solving products all they are doing is selling misplaced hope and exploiting peoples faith.

I would be interested to hear from anyone that has used this on new(ish) galvanising and how long it lasted.

On a similar vein in the 90's I fabricated a cantilever car port from mild steel, I painted it with galvafroid zinc rich paint and no further coating because again paint doesnt like to stick to it until weathered.

It has been up 25 years now and as I have never had to replace the roof sheets I never got to put a protective paint coat onto it, it was fine for 10 years, started showing signs of rust thereafter which should have been my signal to redo it and put a top coat on, after 15 years it looked quite shabby and now it looks very rusty but it is only surface rust, no deep pitting.

It broke down first on the sun exposed and Wind/rain swept upper surface especially the thinner edges where its hard to get a thick paint coat.

When I do get to redo it which will be between tenancies unless there is another hurricane then it will be shotblasted, hot dip galvanised and never painted.

A tip for galvanising, never write on the steel with a magic marker, the galvanising wont take and you will forever see your writing in rust.

[PaulT]

Thanks for all the replies.

Yes, when I posted and mentioned Hammerite I had spotted the straight to galvanised