My suggestion is to (if it's possible without doing damage to irreplacable seals and other parts) take the door apart, strip the front panel to bare metal via soda blasting or glass bead blasting, and then prime and repaint using rattle cans.
If you can't take the door apart without risking damage, then I'd block sand the rusted areas until they are feathered well into good paint. Treat the areas with a rust inhibitor and prime. You can buy high-build primer that will build back that area that you've feathered so that it blends well with the height of the rest of the painted surface. Block sanding between primer coats will help keep it level. Then do a final prime of the entire door. You can start painting the entire door after that and with the door laid flat, you can really lay down a nice layer with rattle cans and have it flow together and shine well too.
