Reddit mentions of SEM 39143 Trim Black Aerosol - 15 oz.

I sanded until my thumb bled.

Then I hit it with high fill primer:
https://www.amazon.com/gp/product/B000PKX16A/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

Then I sanded that down smooth through the thumb pain.

Then I hit it with the texture spray linked above.

Then I gave it a couple of coats of "Trim Black":
https://www.amazon.com/gp/product/B000EM019O/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

The texture spray really hides a lot of the imperfections

https://www.amazon.com/SEM-39143-Trim-Black-Aerosol/dp/B000EM019O/ref=sr_1_1?ie=UTF8&qid=1537665648&sr=8-1&keywords=SEM+Trim+Black

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Tape off and paper anything you don't want to be black. Scuff with a grey scuff pad and spray it. If it's cracking or pitted you may need to go after it with some 400 or 600 grit sandpaper until it's smooth. Prep with a degreaser or IPA before you spray it.

Use SEM trim paint. It's extremely easy to use and gives an OEM matte finish.
https://www.amazon.com/SEM-39143-Trim-Black-Aerosol/dp/B000EM019O

No need to sand down to bare metal if the paint isn't pealing. Just take a red scotchbrite pad and scuff them up real good so the new paint will bite and get good adhesion. Wipe them down with some wax and grease remover, then mask them up to keep over-spray from getting everywhere and shoot some SEM trim paint on them.

I used SEM Trim Black with great results. I used an adhesion promoter first. 1+ year on the flare looks the same as the day I painted it.

The problem with bed liner is that it's textured - and you have to strip/sand it all of if you end up not liking the textured look. But it's great if you like the look!

https://www.amazon.com/SEM-39143-Trim-Black-Aerosol/dp/B000EM019O/ref=sr_1_1?ie=UTF8&qid=1505082037&sr=8-1&keywords=sem+trim+black

http://www.amazon.com/SEM-39143-Trim-Black-Aerosol/dp/B000EM019O

$7 a can