I accidentally spray painted some gates for my deck with interior oil-based paint. They had been previously painted and I was touching them up after about 7 years use. I put 2 coats of the oil-based interior paint on them. They are white. I'm wondering what I should do. Would it make any difference to put a coat of exterior paint on top? Should I just put them outside and wait for them to wear out again?
I have used oil based indoor paints many times outdoors. Mainly on sheds. The only issue I have had with it is an extreme amount of discoloration. Bright blue turned to really dull pool liner blue, off white turned to dirty-light-yellow.
Honestly I would leave it be for now. If you painted over other latex it might just fall off once it fully hardens and hits a few temp changes. That will make scraping it easier. If it doesn't fall off then just see what happens. It isn't hurting anything.
One big difference between interior and exterior paint is the flexibility. Interior paint does not have to deal with as drastic of temperature changes. Odds are after a summer the oil based paint (even if you put a coat over the top) will get very brittle and begin to crack, particularly if the paint did no go onto bare wood.
Most exterior rated paints incorporate UV inhibitors to protect against the sun's harsh rays, while interior rated paints do not. So I'd expect discoloration and premature deterioration.