I apply/bake it on prior to assembly, then install the guard and handle. When the final product is sharpened, the sharpening takes it off the edge. It does have a shelf life.... that can be anywhere from as little as 6 months... to a year, depending on climate/environment. If you ever allow it to freeze (leaving the can in freezing weather in a shop) it's done. If using the liquid and an airbrush, when it goes "bad".... it won't apply smoothly....it'll appear like a bunch of tiny dots on a blade's surface.
The biggest key to it is the application....VERY sparingly. I generally use the satin clear liquid..... applied with a cheap Harbor Freight air brush. Just one or two VERY LIGHT passes per side is all that's needed.....if you apply it until you can see it (as you would "rattle can" paint), then you've applied too much..... acetone it off and try again. Otherwise it will come out looking like the blade has a plastic coating.
If you're intent is to use the "rattle can" version, I'd recommend against it..... the rattle cans apply way too heavy of a coat, usually producing drips and runs. I tried them early on, and until I went with the liquid version, applied with an airbrush, it was a train wreck.
