Just to clarify - you spray it (or roll it), on the outside, back of the tank.

I wouldn't spray anything on the inside either.

I have been trying to do this for some time, however my urchin likes to eat it off the back wall lol.

I plastidipped the back of my previous tank.
I ran it for about 4+ years and it held up fine.

If I recall I pretty much used most of a can on the back of a standard 65. So that's about 6 sf. So I guess you could decently cover 8sf or maybe even 10sf with one can.

If I had purchased a tank that didn't have an option of having the back colored in some way, then I would do the plastidip method again.