Wood fillers

I'm working on restoring a wooden window. Much of the exterior of
the window has moderately deep (.25 inch) checks on the weather side
creating a rough surface. My goal is to smooth the surface without
removing the wood by using a filler. Ideally, this filler should
have durable, flexible, and vapor permeable properties while being
liquid water resistant and could be painted over (UV resistance
would therefore not be paramount). Obviously, these requirements
removes both epoxies and caulks from the list.
Any suggestions?

Jeremy


                                  ***
                  Conservation DistList Instance 16:33
                Distributed: Thursday, November 14, 2002
                       Message Id: cdl-16-33-022
                                  ***