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Where would you find a 2-component urethane? Are you refering to what can be found at ie: Advance Auto, or a professional body shop supply place??

I am aware that bare metal revealed by sanding (or sand blasting) should generally be coated with a self etching primer. From what I recall, is that frames are dipped in a phosphate solution that has a charge put through it, then painted. Not sure about this part; depending on what media is used to "sandblast" a frame, anything hard will also remove the factory phosphate coating, but something like CO2 or some sort of plant material may not. Will have to learn more about that.

But the axles I think are just painted. So, makes sense that if you chemically strip an axle down to the bare metal may have to use an etching primer first, then the top coat which probably should be something that can withstand oil, grease, etc.