I built my darkroom sinks out of 3/4 ply and then had them coated in fiberglass. I then painted them with the best exterior oil paint I could find.... Average paper developers are caustic enough to remove exterior paint after about a month of daily use. I strongly recommend that you use a marine epoxy paint. Even that will require a new application after 4-5 years. This is not overkill. You would be wasting your time and money using an exterior house paint.