I'm trying to add more density to one part of the hair mesh... around the head there is a patch that I can't fill - there is also a larger polygon where it is thin.  Does the add density tool add more hair? Or does the area need more polygons?

Okay, I've made some progress.  I did improve the polygons in the mesh around the eye and it helped with the bald patches.  This is an animation of the beaver swimming.  I'm not happy with the rigging - did it a wile back.  I had to use morph targets to move the head as I rigged it poorly and the  head/neck controls the body.  Anyway - this works for now.  The hair on the face is not moving like I want... Any suggestions appreciated. R

A way to do this is by making a hair density map. This is a gray scale UV texture map you can use instead of painting the density on the mesh. The DAZ tutorial forum has the technique laid out nicely by Holly2000, using milcat as an example.

Yes, I forgot - thank you I'll check it out.  I'll be able to use that - Here is a hair test - I forgot to simulate the light hair so it doesn't react - My dog's hair never looked this clean and bouncy. 

Here is the full fur flying movie.