If you really trust it and you feel its flat enough for you then all you need to do is hit it with a maroon scuff pad. But do some prep first.
By prep in this case what I mean is clean it and clean it and clean it before you touch it with sand paper. Wash it with dish washing soap and hot water (if available) a couple of times and then wipe it down with fresh rags and L-thinner or acetone once or twice too, then wash it again a couple of times. Then it it with a degreaser-wax remover and the - only after you are completely sure there is no trace what so ever of old wax, then scuff it.
The point I'm trying to make is don't just pick up a tank (or other parts) and start sanding on them. Old wax is not removed by sanding, all that happens is it gets ground in. If you miss any you will get a fish-eye, its that simple.
Now, if you were to want to sand it then it depends a good bit on what you intend to do next. But if no metallics or pearls are planned then you should be safe wet-sanding to 400p or maybe 600p if you like, but not any finer than that because you'll start getting adhesion problems if you get it too smooth.
