Looks like they were having issues with the bed liner sticking without a primer.
The price seems like a ok deal for plates. I pay over $1000 a sheet for Austrian armor sheet in 6.2mm thickness.
From a maker point of view, if I'm offering a great price (and this looks like this guy is offering a great price) if someone asked for additional discount, I would get kind of insulted.
My guess it is either a epoxy primer or a primer designed for urethane paint. Personally, I would have just used parkerizing or a textured matte black powder coat.
From their site:
LEVEL III SPARTAN ARMOR - COATED AR500 BODY ARMOR
The Target Man, LLC has spent countless hours and research on developing effective fragmentation/spall protection for our AR500 Spartan Armor steel trauma plates. We have tested various types of bedliners, kevlar sleeves, and plastic coatings and have found our best fit. We are pleased to now be offering Rhino Lined coated plates. The Rhino coating, combined with our non-traditional primer (standard primers don’t work well), has proven extremely effective in containing the fragmentation and spalling during our testing. Click on your desired configuration of our formed plates below to view our demonstration videos. We’ll let the videos speak for themselves.
The hardest part is the forming of it. It is some springy stuff that needs either the temper removed, then bent and re-tempered, or some serious over bending cause when you bend steel (or most any other metal) and you want a certain angle you have to bend it past the actual angle you want, then the metal springs back to the desired angle.
Water jet cutting is pretty standard stuff to cut it. Water jet cutting is expensive though cause those machines are finicky, and require lots of care to operate and maintain.