My experience with POR15:
a) no grease, humidity on the surface is a key - but this is general rule for all coats 😉. Don't use compressed air to "dry" the surface, unless you have the humidity and oil trapping system in your installation. Accordingly to POR15 manual (yes - read it first !), the surface must be completely dry, "or it will ruin your work" (especially when it is not flat surface). Best is to use the heat gun if possible (or hair dryer, or leave it for some time directly at the sunshine) - but don't coat it while still hot, of course !
b) I get the best results when using really really small amount of the paint on the brush - so small that it was looking not possible to paint with it, but then: I was not dragging the brush there and back, but I was making many "touches" - one by one (not sure how to name it in english, but I think you got the point). First it was looking of course not good, but just leave it for some time, and POR15 starts to spread ....
c) the more paint you have on the brush, the worse result is - that is my experience
c) my result below (you can see my fingerprint close to the label, on the left)