That was brave of you to try that with a normal brush and use turpentine as well. I had a bit of a freak out when I did mine. not so much the light gray borders, but spraying the patterns on mine with subtle color differences. It took me several tries to find the right colors and used rubbing alcohol to clean off the paint after each retry. Unknown to me was that the alcohol had traveled up the panel lines and seeped under the masking tape and spread out despite my best efforts to press in the tape and seal it off.
So when I pulled off the tape, thinking I was done, there was a lot of paint that went with it or was all splotchy because the alcohol ruined it. I had to repaint the black and red painted areas and try to fix the SMS logo and "SMS 030" on the wings as best I could by masking and respraying and doing touch ups with a fine paint brush. That was horrifying.
I guess after your ordeal, you aren't going to try to do the lighter gray stripes on the tail fins and the canards?