Aluminum - de grease, sand with BOLL abrasive cloth, blow, and de-grease again with "BOLL silicone remover", then apply the next coat. BOLL SPRAY PUTTY 1.2KG. steel - de-grease, sand with P80-P120, blow, and de-grease again with "BOLL silicone remover", then apply another coat.
eBay