The 'grainy' appearance, technically called spangle, is only produced by dipping a part in molten zinc and nowaday it' s almost impossible to find somebody that does it in a small scale in the US (enviromental problems). In Georgia few years ago they replaced old wood power poles with very large galvanized ones where one can see the spangle very clearly. Anyway, a good advice is to diluite the acid-usually is about 30 % in the stores- to about 5 %, to control the process better.
By the way, that rear license plate appears somewhat satisfied in that photo!