The Catalina also had a good amount of fabric in its design. I believe all of its control surfaces (ailerons, elevators and rudder) were fabric covered metal frames.
I've heared that paint didn't chip off of fabric surfaces, but instead cracked over time, giving what is often refered to as a "crackle" appearance. That's something you may want to research.
I also suspect there was fabric in other parts of the Catalina's construction as well, as I've heard that there were some inspection panels on the tops of the wings that opened and closed with zippers.
Anybody out there who can confirm these things? I haven't researched that particular aircraft extensively enough to know for certain if these things I've heard are true.
As for the grease on the wheel well areas, a bit of gloss black paint here and there, streaked in the direction of the airflow over the aircraft should do the trick, you might also be interested in simulating hyraulic fluid near any hinged areas on the aircraft, faint red steaking is what you want for that.
Good luck, hope this was useful to you
P.S. There was a short research article published in Finescale Modeler shortly afte Monogram first released its 1/48 PBY-5A kit, thats probably about 5 years ago.