My method of prepping and painting the prop shaft is not working out well anymore - and I would love your recommendations for what works best. I have been sanding the shaft down to bare metal and applying a 2-part Interlux etching treatment, and then painting with good ol' bottom paint on shaft and prop both. This has been working fine until the past two years, and I'm getting barnacles.
Any tried and true method you can suggest?

Speed Prop had mixed results for those who used it.

I've tried over the years the following....
Wax & polish, Vaseline, Lanolin, Baby Diaper Cream (high zinc content). All eventually wore off and the barnacles would start growing by the end of July. It's amazing how fast the little ones start to form. This year I sprayed the prop and shaft with Rustoleum Cold Galvanizing paint it has high zinc content. One of the guys I know who races on Hustler uses it on his boat and says it works great. I'll let you know later this season.