I have built four oak wine barrel chairs that will be outdoors in a dry, high-UV climate. I see other chairs in our climate that the poly finish cracks and doesn't last long. I would like to put on an oil finish (Danish or tung) for the health of the wood but don't want to be doing it monthly. Can I put poly over oil if I change my mind? Any suggestions on best finish for my situation?
UV exposure is best dealt with using something with UV inhibitors: paint, stain, or marine spar varnish. Oil finish will not last long at all, ie it will require frequent renewing (monthly in a sunny climate seems about right!). Do not use regular poly, it will degrade and be very unsightly. However, you CAN put spar varnish directly over oil if you change your mind (at that point the oil will basically be gone, or soon enough, anyway). same goes for oil-based stain. Paint can be applied also, but do a spot check of adhesion with the right primer... eg, latex paint won't adhere to a newly oiled surface.