MDF is not intended for high moisture areas. There is a water resistant MDF manufacture, the brand name eludes me at the moment. But I believe the indented purpose is occasional water contact such as a sink leak, not exterior applications.
What profile do you need to match?
__________________ "It looked good on paper, but paper is funny that way"
MDF cannot be used for exterior application. MDO can but would not be a good selection. Use Azek sheets. You can shape it to anything you like, paint to match other trim and it will not rot or attract insects.