A simple flat black is Eastwood DTM etching primer in black. It is pretty durable on it's own. For long term exposure, like a rocket is unlikely to get, it is fine. I use it in my business for restorations when someone does not want to pay for the 2k urethane Rat Rod flat black, also from Eastood. The primer is very low build, so your prep needs to be good. The cans hae a great spray nozzle, and last many uses if care is taken to spray out the nozzle after each use (upside down until clear) On a rocket, for flat black, I would not hesitate to use it. It covers great, but is relatively low solids, as most rattle cans are, so your prep needs to be primo.