I recommend using fabric paint for numbers over other methods for several reasons:
1. Durability and longevity
Painted numbers can peel, scratch, or chip, but this is easily remedied by patching problem areas with more paint and smoothing the surrounding area with an applicator like a sponge or finger. Properly painted numbers are not likely to be damaged to this extent though, as they are bonded to the fabric better than the other two options. Painted numbers are chemically bonded, while the others are mechanically bonded. Iron on are somewhat chemically bonded, but I feel that most iron on adhesion is more mechanical than chemical.
For most teams, iron on is still a decent option. We haven't tried embroidering, but knowing how much we've damaged the bumper fabric itself in the past, I would be weary of my team using embroidery. If you think your team won't mess up your embroidery, go for it.
2. Customization
It is easy to create numbers in any font and stroke (and color if you wish) from painting. Simply cut a stencil and apply the paint through the stencil. Custom iron on numbers or embroidered numbers may be more expensive and take time to manufacture.
3. Easy alignment
Embroidery and iron on methods require that the numbers are applied to the fabric BEFORE attaching the fabric to the bumper assembly. With paint, you have the option to apply the numbers AFTER attaching the fabric to the bumper assembly. I like this option because I don't have to worry about aligning the fabric perfectly when I put it on the bumper assembly. I can instead focus on a tight, wrinkle free wrap. After the wrap, then stencil can be applied in the perfect position for EVERY bumper, so each side is consistent with the rest.
Aligning the numbers on reversible bumpers is also easier by painting. The other benefit is that paint can fold easier than iron on and embroidered numbers.
4. Aesthetics
When done right, painted numbers can look just as good as iron on and embroidered numbers. (see pictures)
Here's a closer look off the field:
You just need to take time to stencil the initial lines and then use masking tape to keep the edges nice and crisp. If there are curves, cut the tape with a razor blade on a separate surface. If you prefer to not use tape, you can print a number onto a sheet of frisket paper and cut it out. Frisket paper is sticky on the back, so it serves as both a stencil and adhesive masking.
We only use our stencil for the light coat of spray paint that goes on the fabric. We then remove the stencil and use the very faint number as the guide for our masking tape. Then we add about five thin layers of fabric paint to make the numbers durable and 100% opaque. One thick layer is not as durable. We don't apply the paint in strokes, but rather in dabs with sponges to avoid making the numbers "look" painted. It makes for a nice matte and flat finish.
If anyone has questions about our process of making perfectly painted numbers, just ask.