I noticed something similar to this recently. It only shows up on the application of the second coat of sensitizer. When you go to put the brush down for the first time, you need to make sure that you place it into the middle of the puddle of solution, not on a dry portion of the paper next to the solution.
The water in the brush will start to dissolve the first coat a little if you don't get it into the solution, and that can result in an unevenness in the final print. If you start by dipping the brush in the solution, then the water is not an issue, and everyting will go smoothly.
When I do test strips, I use a large piece of paper similar to the size of the print, and then cut strips out of the paper. this eliminates some of the variability associated with changing the coating size, etc. This is one case where I disagree with Dick's book, because I don't think a small test strip coating is ever going to be equivalent to a large sheet coating. They are not scaleable in a predictable way.
This is a good reason for people to sort their paper when then get it. If you have paper with defects, these are the logical pieces to use for the test strips. When I cut a large piece down for a 7x17 print, I am often left with a scrap of paper and two banquet sized pieces. I sort as I cut down, and if one of them has a defect, then it goes into the test sheet pile. The scraps are actually large enough to use for tests also, so I manage to have little waste.