Re painting an aluminium casting.

[KAM]

Anyone got any tips for repainting a small finned aluminium casting. Last time I tried it was very difficult to remove the oxide powder. It looked clean and good after epoxy primer and top coat but only lasted a few years. Just wondering if there is a chemical wash I can use or maybe it's something an alloy wheel refurbisher could do. Advice appreciated.

 

[wallacebob]

Acid etch primer. Car places sell alloy wheel refurb kits.

 

[vyv_cox]

Or my favourite - Hammerite Special Metals primer followed by any decent paint such as their smooth ones.

 

[KAM]

Got phosphoric acid. That should help.

 

[vyv_cox]

The etchant is not acid, it is alkaline, typically potassium hydroxide. Some aluminium alloys are barely attacked by acids

 

[KAM]

Never been very impressed with Hammerite. Epoxies seem to be so much better. It's amazing the volume of corrosion on aluminium castings. Having brushed most of it away its hard to see what has corroded and where it's all come from. Slight drift but the main bearing on the winch I'm refurbishing had completely disintegrated. No balls left. Just wondering if people have felt it worthwhile replacing standard bearings with stainless units now these are readily available.

 

[Zing]

Alodine, aka Bonderite is the go-to primer. Clean and prime properly are the key steps, pardon the pun.

 

[geem]

A friend on mine built a 57 aluminium yacht in 1981. He has been sailing it ever since. He has had to deal with a lot of aluminium corrosion issues over then years. Clean it with what ever you want but once you have got the aluminium to bare aluminium and clean, do nothing for 3 days until it has an oxidised layer before painting. This is his advise based on experience. The next time we need to do our steering pedestal I will give it a go.

 

[Fr J Hackett]

Aluminium oxidises very quickly in fact you will never see an unoxidised surface in normal circumstances, the hard oxidised surface is what is responsible for aluminiums corrosion resistance it is when the surface film is destroyed and cannot repair itself usually due to acidic or alkaline environment or the close association of another metal that corrosion takes place.

 

[Halo]

as vyv said hamerite special metals primer is amazing for ally etc.