cleaning stains out of aluminum engine cases?

Paul, why not fight fire with fire and try a test patch with some battery acid?
As soon as it starts to work you can easily wash it off to neutralise it.
It's just a (much) stronger acid than Phosphoric. Just don't leave it on for hours.
 
Anyone in your area do vapour blasting Paul ? Basically wet bead blasting which leaves the castings looking like brand new and (in my experience anyway) leaves the alloy pores closed. Very popular in jetski repair circles and I assume snowmobiles too.
 
Hi Paul,

I tried WD40 and penetrating oil seperately.
From memory the penetrating oil was better as it acts like a cutting agent, but nothing wrong with WD40 if no penetrating oil around.

The oils in these liquids mix with the debris caused by the scotchbrite pad and create a fine abrasive slurry, as I said it makes a durable linear matt grey finish all over - very similar to a lapped surface.
 
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