What will happen if interior paint is used outdoors and it has several coats of polyurethane on it?
The polyurethane will probably crack. Plus, adhesion will be an issue. If you have to use the polyurethane use a product that is recommended for exterior use.
On the paint side, interior paints typically dry harder and can't stand up to temperature extremes and exposure to water as well as exterior paints.
It is best to use an exterior paint.