I've found you can use the epoxy mix from some of the resto vendors like Eastwood , fill in and repair any cracks or missing chunks and lightly sand it smooth . The whole wheel needs to be sanded carefully - I used 400 grit wet/dry and cleaned it with a spray can product called Prep , do not use acetone or aggressive paint thinners as they will damage the wheel's surface . Once it's perfectly clean , hang it up and give it 2 coats of Eastwood's Diamond clear high gloss . Have done a few garden tractor wheels that are really rare and spend a lot of time in the sun - they look new and have held up over 5yrs now .
Sarge