Five years later, the digits I painted with Rust-Oleum Rusty Metal Primer have weathered pretty well, while the original ink has fallen off the retroreflective sticker:
As before, I wiped off the crud with denatured alcohol and painted neatly inside the lines. The other digits on both sides still look as good as the day I painted them, with only a few bubbles and nicks.
Memo to self: Next time, buy a big sheet of 3M retroreflective film, make a stencil by vinyl cutting, paint the entire number in one shot, and be done with it.
The Smell of Molten Projects in the Morning 
Why paint instead of sticking on the vinyl numbers?
I have paint (well, primer) and I know how to use it! [grin]
Even though the vinyl stuck to my tea mug a year ago is (mostly) still there, I’m unconvinced it’ll survive outdoors for very long. Better to use vinyl as a mask, slather on some paint, and peel the mask off in one operation.
I think so, anyway …