So after about 10 hours of scrubbing with acids and alkalis of various strengths I managed to get my block back to what looked like a nice surface with the odd bit of corrosion. Rinsed it all off with clean water for ages, dried and painted it with matte black enamel paint. Oxides show/grow through. Any ideas? I could use a nice thick gloss paint and cover it all but a) I don't want a gloss block and b) it'll carry on underneath.

Anyone found a way of properly getting rid of alu oxides without mechanical removal? I can't physically scrape all of it off due to the casting shapes.

dry ice soda blast is the way to do it and it looks really good

http://www.youtube.com/watch?v=YI_Bnl6l-OY

I have a grit blaster but it isn't helpful in this case as it is built up. Soda or dry ice is nice but don't know anyone around who will do it and transporting the parts isn't going to happen!

DIY soda blasting as in the link!

This is what I have so far!

Description

Probably a silly question but did you use an etch primer before painting it black?

No, because the etch primer i would find was deigned for steel and the paint i used claimed it could be painted directly onto metal after a cleaning.

[Edited on 15/6/12 by coyoteboy]

now, i'm no expert, and this could be wrong, but acids are oxidising agents, so perhaps the acids you've used are causing a reaction later? a stainless steel wire brush would be better i'd have thought?

Indeed, this iswhere my problems stem from I think. All the advice I could find said to use acid to remove it a per alloy wheel cleaners. It didn't help and instead I tried concentrated sodium hydroxide which is used to remove anodising (Al oxide) but that didn't help either. So I rinsed it all off for ages.with clean water. The NaOH cleaned off the surface stuff but didn't touch the heavy stuff. Tried a wire brush but can only get to larger surfaces and it's the tight stuff that is worst corroded.

[Edited on 15/6/12 by coyoteboy]

I was hoping there'd be a few with experience on here Looks like I'm a bit stuffed.

You need to do it with an etch primer suitable for aluminium, the chemicals that you cleaned it with would have removed the original oxide layer but it re oxidises almost immediately when exposed to air. The etch primer has an acid component that eats into the alloy and then the paint keys straight to it before it can oxidise.
I had a cam cover soda blasted and then painted it with engine enamel and found that it had started to flake after a few months due to no primer, the alloy panels on my car have been no problem as I used an etch primer .
Paul

AUTOTEK PROFESSIONAL GREY ACID ETCH PRIMER – ALUMINIUM, GALVANISED METAL PRIMER | eBay

This should do the trick

Cheers folks, much appreciated. I'll grab some alu etch primer and see how I get on with the heads and sump areas. I'm not sure where I'll go with the bit I've painted as I can't strip it very easily now. What a pain.

Thanks again