Easiest and most durable is to find a powder coating outfit and take it to them, can be a bit pricey though. Below is a powder coated enclosure with art work on a water slide decal:
I normally use etch primer followed by enamel paint both from aerosol spray cans. For any lettering etc. I typically use designs printed on water slide decals. I then clear coat with a bunch of coats, again aerosol spray. This finish is pretty hardy but will tend to chip on corners if dropped. This finish is quite a bit of work. This one is painted:
An easy and robust finish is to sand and polish (optional) the enclosure and then etch any markings you require. I use a mix of hydrochloric (muriatic) acid hydrogen peroxide, the same mix I use for etching PCBs.
Rob.
