Hello Redpiano:
I think with the Radiator grill, before applying the Eduard PE, the modeler is supposed to smooth out the existing radiator details. It is frustrating. I would tape the stiching and sand, you are almost there. On the underside, tape off the details on the underside of the wing, fill in the seam and sand until smooth. Like Chris said too, sometimes just walk away, or put it back in the box, work on something a little less "fiddly" like an Albators or the Sopwith Pup, and come back to it at a later date.
Good Luck
Mark