The best way to start is to sandblast the sign. That will provide the best finish for repainting. I have been using aluminum oxide for my blasting media. I rarely use primer on my cast iron restorations. I have found it is not required. I use Rustoleum brush-on oil based paint on most of my outside railroadiana. I have also used POR-15 paint on a couple of pieces. However, it requires more prep and application time. It is also expensive but is supposed to last much longer than other paint systems. Most of my Rustoleum painted pieces will last 10 years on average outside before they need to be repainted.
Here are some of my restorations.
