Painting aluminum

[blackrose]

I'm just primed my stem deck with jotamastic 87 epoxy after taking it back to metal. I want to paint the aluminium generator locker and was going to etch prime it, but where the epoxy deck paint has gone over onto the aluminum at the bottom it seems rock solid. Can I prime aluminum with epoxy rather than an etch primer? I didn't think that was supposed to work? 

[Boater Sam]

It will probably flake off if there is no etch primer when the aluminium gets its surface corrosion back.

[blackrose]

4 hours ago, Boater Sam said:

It will probably flake off if there is no etch primer when the aluminium gets its surface corrosion back.

But how could the aluminium get its surface corrosion back if it's been painted and there's no oxygen on its surface to react? You might be right but I think I'll key a bit more, paint it and do a proper test. 

[Jen-in-Wellies]

Surface corrosion on aluminium takes place almost instantly on exposure to air. It is a thin aluminium oxide layer, strongly bonded to the underlying  metal that then inhibits further corrosion. It looks like uncorroded metal to us. Anodised aluminium is a process where the oxide film is deliberately made thicker. Generally aluminium takes paint well. Anodised takes it better. Shouldn't need an etch primer. Just clean, degrease and dry the surface. Bad corrosion of aluminium is mostly when it is in contact with other metals, eapecially steel fasteners. The aluminium acts as an anode and corrodes, protecting the steel. Over time you do get the white dusty corrosion  products on the surface of aluminium, but the underlying metal is sound.

 

Jen

[blackrose]

Everything you say seems correct apart from not needing an etch primer? I once painted the new aluminium canopy of a passenger boat in sections by abrading, spirit wiping with the thinners for the primer and then applying an etch primer within 20 mins. So I thought aluminium did require an etch primer otherwise it wouldn't last? My original question was about epoxy because it appears to work, but whether it lasts I have no idea. 

 

https://www.austenknapman.co.uk/blog/faqs/a-3-step-guide-to-painting-aluminium/

Edited July 20, 2018 by blackrose

[Iain_S]

On 19/07/2018 at 06:59, Jen-in-Wellies said:

(snip)

The aluminium acts as an anode and corrodes, protecting the steel. Over time you do get the white dusty corrosion  products on the surface of aluminium, but the underlying metal is sound.

 

Tell that to my Landrover!      

 

[George and Dragon]

4 minutes ago, Iain_S said:

Tell that to my Landrover!      

 

Birmabright is a special case perhaps although, to be fair, once you point out to MOT testers that it's the aluminium that's corroded and the seat belt anchors are on solid steel they have to look for another reason to fail the vehicle.