Before you
can effectively tackle the problem, you need to know what sort of enamel it is and what underlying material(s) are involved.
Since chlorine bleach didn't help, there's a strong chance that you are dealing with an acrylic enamel. I've never read of any way to rejuvenate acrylic enamels. Is the enamel covering only steel surfaces or is aluminum or plastic involved? If you're only dealing with steel, then stripping the enamel and repainting is easier than any stop-gap solutions. If aluminum or plastic are involved, then stripping becomes very much more complicated. It is better to lightly sand/fill and repaint.
There are several excellent guides on the internet to repainting cars using rollers and thinned enamels. The results are excellent and probably easier to achieve on large vertical panels than with spray paint.