If you're just sitting in an office & not doing anything, i.e., waiting for a phone to ring or some kind of job where you have lots of idle time, so the boss is paying you to just sit there, then they might find it ok for you to be doing something of your own during your "down time;" but your 1st priority is to the person/company that is paying your wage The only way it would be ethical is if you ask your boss if it's ok and explain exactly what you would be doing and what of theirs you would be using. Surely they would at least want you to pay for materials used. If you don't ask then the answer is Abolutely Not!!