Filters and filter papers are a very commonly used laboratory supply in a variety of laboratory settings. I know that through my own experience and in discussions with other scientists, that the majority of people tend to rely on Whatman for the majority of their filtration needs. However earlier this year I learned of a company called, Munktell, through one of my primary lab supply vendors. I requested a sample package to do comparison studies on the Munktell filters along side of the Whatman brand 541 papers that I had been using. The papers passed my validation study and I have since been using them without any issues. In fact, they actually seem to resist retaining moisture from the ambient environment better than the Whatman brand papers that I had been using. I have also noted that the Munktell brand are always in stock whereas I was having issues with the Whatman papers being on back order many times, which caused very bad turn-around-time on orders, this often causing me to get alarmingly low on the filter papers.
I just wanted to pass this along to others as you may want to consider giving Munktell a try for your filtration needs. From what I have seen, they tend to run about 20% cheaper than the Whatman brand. So, couple the better price point with the better lead time and I think you will be very pleased with this company, should you switch.