This article covers simple black house numbers on a white background or white house numbers on a black background.
Use a wire brush to knock off everything from dust and dirt to surface grease and grime. A stiff wire brush can begin to wear away the surface of the curb with some effort. That may be a good thing if there's road grime. A fresh surface will hold paint better. Don't try to dig too deep, just remove the surface crud.
Use a soft bristled scrub brush to finish preparing the surface to remove any dust that remains. Newer curbs won't require much preparation and may appear clean but you should still use the wire brush/scrub brush combination to get your curb ready for paint.
Using 2" masking tape, create your background area. Since I recommend and use Cole brand numbering stencils (made by The Hillman Group), I know that a 3" high numbering stencil is 2-3/4" wide. If you are painting a curb number that has 4 digits, the total amount of space you'll need left to right is 11". I always add an inch to each side so the numbers don't appear crowded within the background. So the width of the background for a 4-digit curb number is 13"
The bottom of the background looks best at about 1" off of the street surface, so I run a piece of tape to create the bottom of the background. 3" Cole numbering stencils are 5" tall, and after trial and error have found that a 5" tall background works very well. Run a piece of tape 5" above the bottom edge piece.
Your background should be formed now. Stand back and make sure your placement is right where you want it and everything appears nice and square. Measure your background on one side, then the other to help ensure it's squareness.
You can tape some old newspapers outside the taped area to prevent over spray.
There are a multitude of different kinds of paint available, but the reality of curb numbers is this: If you can get them to last about 5 years, you've done very well. There are permanent paints available that claim to last over a decade, but you will need a spray rig to use them and the final paint job will still begin to flake apart after about 5 years anyway. You can also buy pavement striping paint. It really does last a long time... but again, you'll either need to brush it on or get a spray rig and the thickness of striping paint requires lots of drying time between coats.
The best, most reasonable paint to use for painting address numbers on a curb is Rustoleum Flat Protective Enamel. You can buy it pretty much anywhere, it's inexpensive, it lasts a long time and it comes in a spray can. Be sure and use "flat" paint. You can paint the background white or black and use the other color for the numbers. Follow label instructions for general use and paint the taped off background.
Lay your stencils on a flat surface facing up in the correct order you want them, lining them up at the top and at the bottom. Butt the stencils together at the sides. Tape the stencils together using 3/4" masking tape. Extra tape beyond the stencils will help you tape the unit into position.
You can tape your stencils into place after just a few short minutes or until the background feels dry to the touch and tape will stick to it.
From top to bottom, the stencils should fit nearly perfect into your background. From side to side, you can easily find the center of the background for placement. Once you have your stencils in position, run a strip of tape all the way around your numbers, making sure your background is completely covered up from getting sprayed.
You may want to wear rubber gloves because as you spray the numbers, you can (and should) press down the stencils as you go to avoid as much under spray as possible. Curbs are rough and under spray is a common issue. Take your time and do a good job. It only takes about 1 minute to spray 4 numbers carefully. Use short bursts and hold the can about 5-8 inches away from the stencils for the best results.
Once you finish painting the numbers, you can begin to remove each layer of masking until everything has been removed. Your curb number is basically done. Once everything has been removed, you can touch up the background or the numbers by spraying a little bit of paint into the cap of the spray paint and using a hobby brush to touch it up. You'll need paint thinner and a rag to clean your brush.
Now you're done!
In my experience (and I've painted a lot of curbs) address numbers painted this way will last between 5 and 7 years. In my handyman blog, I have a couple of photos of a curb I painted right after I did it, then again 4 years later. See how curb numbering looks after 4 years here.