The key to keeping a boat looking good, with no oxidation, is to keep it polished and wax on a regular basis from when it is new. Regular mantenance at least two to three times a year minimum.
Once you can see oxidation on the gelcoat it's usually too late to do anything to keep the oxidation from reappearing. And yes, on a boat that contains heavy oxidation, it is possible, and usually very likely to actually begin to see the oxidation reappear within a few days. That's because the oxidation is coming from within the gelcoat itself. Of course there are a number of good products on the market that can be used to make the surface look clean, smooth and shiney, however that good looking finish on a boat that contains any amount of oxidation usually does not stay looking good for very long.
You can think of oxidation coming from within the gel coat of the fiberglass, similar to the way rust comes from within the surface of metal. If you were to take a piece of metal and grind away the rust and then coat it with car wax you would probably see the rust begin to bleed thru just like the oxidation comes back on an oxidized boat.
From a business standpoint, my basic rule is if the boat is over five years old and it has not been polished and waxed on a regular basis from when it was new, then I will not take on the job. Generally speaking, if you see oxidation then it's a sure sign that the boat hasn't been taken care of on a regular basis.
The key is polishing and waxing on a regular basis from when the boat is new. That's the only way to keep a boat looking good.