Whatman has a whole chart on porosity telling if the filter papers a fast,
medium fast or slow, extra slow. There are also rough versus smooth filter
papers.  There are equivalents sold by companies under their brand name,
VWR, Fisher.  Catalogs often give filter paper information and on the back
of many boxes, you can see a comparison chart.  Example is Whatman #54 is
medium fast as is #4, one had hard smooth finish, other is rougher.  

One hint given years ago was to not use slow papers for filtering some
stain solutions as the dye can be retained on the filter paper surface.
Medium fast was recommended. 

Filter papers are pretty heavy duty, I vote for the PERM papers, very
similar to good quality lens paper. 


Gayle Callis
MT,HT,HTL(ASCP)
Histopathology Supervisor
Veterinary Molecular Biology
Montana State University - Bozeman
Bozeman MT 59717-361