[SAJ]

I would just sand the lumpy spots smooth, looking for cracks or holes, and if none are found, hit it with black enamel aerosol. I use Plasticote gloss black enamel aerosols. On the muffler I sand lightly, hit with phosphoric rust converter, wipe off with a wet rag when reacted, and then spray with black Plasticote BBQ paint.
I do this on a hoist every few months using a piece of hand held cardboard to stop overspray.
I just want the underside to look clean and neat, but not to pass a inspection, since we drive a lot on gravel roads so there are always chipped areas needing touching up.
Using a spray gun involves too much work, specially cleaning it afterwards. The aerosol is the work of minutes literally. I always finish by spraying inverted into a waste bin to clear paint from the nozzle so it is ready for action next time.
SAJ in NZ