Hello diver30. If you'd like, you can repaint it - it's up to you, but concider first:
0) Are the tanks nice on the inside. Maybe the inside is rusty / damaged too. Then you will have to fix that first.
1) your LDS may not want to fill repainted tanks before they have been pressure tested, maybe not even then.
2) repainting it is - as Fred pointed out, pretty much work, and will cost you some if you do not already have the stuff.
3) If you're not doing it really professionally, the new paint will note last very long. In the factory, the paint is usualy "burnt" onto the tank in an oven. This method makes it more durable. However, do NOT do this yourself, as heat may cause tensions in the material of the tank. It could become a potential bomb.
I repainted a set of doubles two years ago - same method as Fred. It turned out great, but I wouldn't do it again. It's simply not worth the time (in my opinion), since 10 l tanks are pretty cheap.
Regards
/Kristian