Using dry portland cement mixed with latex paint primer as a filler for weathered wood
I doubt that will go well.  Wood expands and contracts a great deal due to changes in temperature and humidity.  Cement doesn't, especially portland cement.  In addition cement doesn't really bond all that well to wood: that's why it's commonly used to make forms.  My guess is that the cement would pop out within a year, if not less and make it look way worse than now.

There are products specifically made for this purpose.  You should use one of those instead if you want to do this.  Use one made for outdoor applications.