Q: How to clean an aluminium Intel iMac.

Hello,

http://support.apple.com/kb/HT3226

https://www.klearscreen.com/iKlear.aspx

The trouble with aluminium is that some stains sink into the metal.   The first two things you should consider ...

Check out apple support ... cleaning my Mac.

Google the term Aluminium Associations and from the list you may find someone with technical expertise to advise you.   But beware; Apple may have applied a surface coating that reacts unfavourably to some cleaning processes.

I assume you are in the UK by the timing of your post.  If so, there is a cleaner on the market here called Astonish.   It is meant for stainless steel.   But as I said in my original post ... Beware.   Try it on an obscure corner first.   Incidentally, Aluminium does not rust.