Cutting powder coated steel - how do I prevent rust?
Rustoleum 7769 Rusty Metal Primer is my go-to for outdoor things.  The cleaner they are, the better it works, but it works with wire-wheel levels of rust removal.
I'm a &quot;prosumer&quot; painter.  I know how to handle the dangerous stuff like LPU, and I don't mind paying $300/gallon if it means I'll never have to paint it again.  I've delved deep into http://corrosion.ksc.nasa.gov and I'm willing to use the mil-spec 2-part chromate primers (the green stuff on new airplanes and locomotives).  That's the only stuff I've found that will beat Rustoleum 7769 on clean metal (as in SSCP-SP10 near-white-metal prep).  For worse prep (e.g. wire-wheeled) I simply haven't found anything at all.
The fact that Rusto 7769 is readily available in the consumer channel just makes it all the better.
Keep in mind my criterion is that it doesn't fail, until the topcoat above it fails.  I prevent topcoat failure by using LPU topcoat, but you don't need to do that.
I would dab it onto the cut edge with the cheapest natural bristle brush at the hardware store, e.g. an &quot;acid brush&quot; (~3/8&quot; wide) or a cheapie &quot;chip brush&quot;.   If you bought a spray can and realized what a mess that will be, and want to brush, simply shoot a little spray into a soup can (cleaned out and dried, obviously) until you have a tiny puddle of liquid paint.
Rusto is an &quot;oil based&quot; alkyd, and it will stink. There's no water-based product that will do what you want.  (least not in the consumer tier).