Hi Nick, glad it worked OK for you. if you really want the ultimate finish you should use a finer grade compound after the G3, theres g10 or 3m finesse it. both excellent products. to be honest there aren't really necessary. Yes you should polish it after compounding to achieve the best result.
Lacquer isn't required if your happy with the finish. some say lacquer will give you a more resilient finish, but I'm not convinced of this. Maybe if you used 2 pack lacquer it would, but you need professional equipment to get the best results using 2 pack. spray can lacquer will have about the same hardness as the spray paint you used already, so I don't see the point if you are happy with the result you already have.
If you do decide to lacquer it, flat it down with 1200 grit wet & dry used with plenty of water & a little washing up liquid. wash it off, then dry it throughly. then clean with pre paint, spirit wipe or something similar to remove any traces of grease etc. white spirit will do the job OK if pre paint isn't available, but sometimes it can melt the paint slightly so use caution & only do small ares at a time & most important of all KEEP YOUR GREESEY FINGERS OFF IT !!!
But first of all see what it looks like after a G3 & a polish. Good luck
Cheers Glenn