Since I can be a bit clumsy with my technique, every once in a while I will tip the knife as I lift off or as I approach the surface and the corner of the knife can dig into the smooth spackle or even gouge the adjacent foam. So I take some fine paper on the small oak block and sand a little radius into the corners of the knife like this.
It doesn’t totally solve the problem (me), but it does seem to reduce the number of occurrences.
Not sure if these pics show the differences well enough between before and now, but this is after: filling the little gaps between spars and foam; building up the areas at the corners of the roof vent; building up the largish low areas in front of the first spar, and between the second and third spars; hatch seams and license plate recess work; various other dings, dents and along portions of the roof edges (all last night); and then sanding it all back down this evening. I still have to do the largish area at the top middle of the hatch, but I had run out of spackle and just concentrated on sanding tonight.
I went thru a whole tub of spackle, but like Karl says, most of it ends up on the floor. The roof vent corners seemed to come out fine with just a little touch-up needed.
Tomorrow will be round 2 of major roof spackle and perhaps get back to rounding the profile edge over.