Agree with RW on this, oil on the outside just creates work, constantly keeping the dust off and repainting would be a nightmare !!I like to stack about four light coats, and I prefer not to spray down the outside, it attracts dirt and once you do, you can't touch up the paint anymore. I like to keep a spray can handy and once a year or so, hit the areas around the firebox that start to burn off.
Some people will swear on spraying the outside down, it's just not for me.