Very "low resolution" painting lines with Substance Painter

Hi Everybody, I'm Andrea from Italy.

I have a strange behavior with SP 11: I'm texturing a jet fighter model I modeled in Cinema 4D and theh exported in FBX format. I perfectly finished the "colors" texturing and now I'm starting the details. I created a new layer in order to manually create the fuselage panels details...I created a fill layer (with only "height" enabled, I set a negative height), a black mask in wich I paint with a very thin brush (0.01 with hardness set to 1)...the problem that all the lines I draw (I set the first point and then press SHIFT to place the second points) are ALWAYS heavily "pixeled", I can't draw something similar to a straight line at all....I tried severa times also increasing the number of poligons in my mesh but nothing changes....the model dimensions are about correspondant to the dimensions of a real small jet plane....the strange fact is that if I create a primitive in C4D having about the same dimensions even with a lower polygons number and I export them in SP I can create reasonably good lines on them without any problem....Surely there's something wrong with my model but I can't figure out what....I know I've been very generic in this post but I didn't know what other details I could provide in order to make you understand my problem....

Any one who could try to help me?

p.s.

I forgot to mention that I unwrapped the UVs on C4D