I use Autotek acid etch primer which is suitable for priming aluminium, galvanized, zinc coated steel, and for GRP and rigid plastics. I've also used it on Bakelite cabinets when I've repaired damaged ones and have resprayed them, such as KB FB10s off the £5.00 'basket case' pile at BVWS events. Great stuff, and it's high build too, so will fill imperfections to a degree.
Single cans are a bit pricey due to postage, but two 500mL cans are typically £13.95 post free from bodyshop suppliers, though admittedly perhaps more than what most would want: EG:
https://www.ebay.co.uk/itm/183718066...xoChVQQAvD_BwE