Brightening the exterior finish

As others have said if the surface is highly oxidized you have to remove several thousandths of gel coat to get the shine back. If hand rubbing will not work I would try the Barkeepers Friend first on a small section and then a hand rubbing compound to see if that brings back the shine. This worked for me for several years. If this does not bring back the shine the other options would be to use a buffing wheel with rubbing compound first to remove the oxidation then use a wax. This is a lot of work and if you have good shoulders is probably the way to go. The last option is to use the Zeps floor wax(if you have bad shoulders). I have used both methods but prefer the Zeps but I am old with two repaired rotator cuffs. I have not had to wax MH in the last 3 years and it still shines.