Thanks everyone! Just found out Eastwood makes a black chassis paint (satin coating) that dries to 25-30% gloss. Just ordered some and will give it a try. I don't mind experimenting at this stage. These firewalls are really ugly with dimples, spot welds, sloppy seam sealer, etc. The shine (even 30% gloss) makes all these imperfections show up even worse, but I want it to be close to factory 'look' as possible.
https://www.ebay.com/itm/37324283804...8AAOSwAXBhA7R4