Paiting the steering rack

[voucht]

Hi,
I cleaned my Escort steering rack to re-paint it with black Hammerite. But I discovered that it is not 100% steel : I tried with a magnet and it doesn't hold on the the gear housing which looks to be in cast aluminium (?).
IMG_1510_1.JPG
I read that non-ferrous metals need to be primed before painting with Hammerite. Is that really needed, or do you think I can paint directly the whole rack?
Second question, I'd like to have my front upright sand blasted. Do they need to be dismantled first and rolls removed, or can I make them sand blasted like that without the sand damaging the rolls ?
Thank you for your help.