Your biggest issue without taking anything off would be overspray.. You'd have to do a lot of masking things off in order to keep it clean plus its harder to prep things when you cant do the entire piece. My dad was a painter for 40 years and has taught me a few things so i definitely think you could do a pretty good job without taking it all apart its just gonna take a bit more prep work. Just make sure everything your painting is as clean as you can possibly get it... obviously the dirt and grease but also things like oil from your fingers can really affect how well the paint will sit up on the metal.