If you like to paint old bikes and you are doing it for a hobby then go for it!

Personally, I don't paint my bikes. I have neither the skill nor patience. I don't even touch them up because I've learned the hard way that it is easier to make the bike look worse than better. Instead I clean the frame thoroughly, sometimes derust it with oxalic acid, and then protect it with car wax. Seems to hold up pretty well.

I also avoid buying repainted bikes, usually because I either don't trust that the bike is what the seller says it is or because the seller wants more than I'm willing to pay for a bike in "perfect looking" condition.