Ok, having done as much research as I felt comfortable with, I came to the conclusion that my aspirations to get the finish close to the original factory finish was just not realistic.
This was more to do with the practicality and timescale to get the job done. I didn't want my car blocking the way at my local indy (D&G Porsche) who I trust implicitly with the work (I didn't have the time or inclination to tackle this as a diy project).
The cooling fan is made up of three component parts.
The fan housing is a magnesium alloy that was factory finished with cosmoline (a bronzish finish).
The fan impeller is an aluminium alloy, also cosmoline coated.
The centre plate of the impeller has an alloy rivited (6 off) fixing which, I think, is a titanium alloy.
Most owners simply powder coat the housing and the impeller with the centre plate intact during the powder coating process.
I was not happy to do this so had the six alloy rivets drilled out.
It seemed the option to carry out media blasting followed by ceramic pait spraying (as carried out on guns and rifles) was simply too complicated for my short time plan. Cerakote is used in the States but I couldn't find a local sprayer.
Much information came from owners in America (no suprise that gun sprayers are more numerous over there) and a chromate finish was the preferred option followed by a satin clear finish.
These are pictures of the before.
Cheers, Keith.