Used a product called Metalfix 100. Water based phosphoric acid rust converting primer, which sets very strong, is heat resistant to 1000C, impermeable, and can be painted over with most any paint if required. Needs surface rust to bond to, which saved me a lot of work. Brush application. Painted my engine bay, firewall and inner guards etc with it. Painted over with high build and satin acrylic and looks great.
The second photo is the straight metalfix, which is how I’m leaving the underside. It has dust and overspray but just a matte black when finished. There is also a red lead colour which is cheaper but not as heat resistant.
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