Keep in mind I did check with the company that makes Sea-Clear, and Clarity plus acrylic aquariums.."The Casco Group"
http://www.casco-group.com/USFS-seaclear.html
before doing this, as the first one I did was a 100gal. Acrylic 9 month old aquarium..
I did forget to mention to make sure you read the directions on the product you use, as no one will warranty it if it goes South, and like Krish7 said, I don't know how hard it is to get it off.
Maybe one of the acrylic tank manufactures might chime in on this one, (some type of acid, I heard once).. The temperature of paint, acrylic, and the work area are a good thing to keep in mind. Also the right preperation of the substrate to be painted is the key. Good lighting is also a factor. All I did was wash the outside with soap and water, rinsed very well,
I used a soft hand towel with hot water to wipe it down, dried it well, mad sure there were no smudge marks, dry compressed air , a couple of REAL light light "tack" coats, then applied enough top coats until I was satisfied..
No runs, No drips, No errors!!! I have heard of some people using a roller and brush to paint the back of glass tanks, dunno it might work, I just kind of like things tight!
Jeff