First off, your not in as much trouble as a lot of people here at the nation would like you to believe.
What you shouldn't do is sand it. That is a poor method for removing anno.
You should get it blasted with 120 grit aluminum oxide to keep from getting particle scratch and it will leave an almost invisible but paint friendly texture to the metal.
Then you'll either want to have it duracoated or cerakoted.
What you heard about regular spray paint is right.
It sucks for paintball use. For one, it will rub right off in short time and it will not hold well the first time a little splatter gets on it. It will just scale off and leave your marker looking like Freddy Krueger.
Anyhow about your options....
Both duracoat and cerakote coatings are like paint in application, although both must be applied with an airbrush.
Duracoat is a type of epoxy paint, chemically bonded with separate hardeners you must mix in right before application.
It an intermediate process that can be learned pretty quickly.
There are multiple companies out there that offer the service of painting them for you, including me.
The colors selection is very wide and the duracoat its self is reasonably cheap.
The bond is very good hence you prep the media right before you spray it.
It is as good as anodizing in my eyes, yet offers the option of unlimited coloring, including matte finishes at a fraction of the cost over all for what you can do with it. Sky and talents the limit.
Cerakote I am not as familiar with, however I do know a few things about it.
Its more expensive, but it is the strongest by far, far surpassing even hard coat anodizing in strength and abrasion resistance, thus making your marker better in durability that is was to begin with.
Its the best of the best, but your going to pay for it.
There are fewer companies that offer this service, but look out!
Expect to shell out a good $5-600 plus for the application for something sized like a small paintball marker in one or two colors max.
Although that is not the limit of colors, just more than you would already want to pay. Its serious business, but it will outlast your marker point blank.
So hope this helps, as beside these you would just want to get it re-anodized.
A few example pics for you...
Yep, pretty plain. Like I said, there aren't to many people out there that even really know anything about it. You will find the best pictures in the gallery of the Ceracoat website I left above.
They copyright everything, so I couldn't post them.