They are right. The full scale planes don't have the horizontal stab attached to the fuselage, but is supported by tubes that allow the horizontal stab to be moved for trimming the plane. Nitrate Dope will not hold when you allow the fabric to make the 90 degree corner from the fuse to the stab. You can either make a cut joint and finish it off with seam tape as is shown in the photo, or make a fairing that is not covered by the Koverall. I too have used this stuff for years, and I have one model that is close to 15 years old that is covered with Koverall and Sig Nitrate/Butyrate dope. It's tighter now that it was when new. Eventually it will split.

Bill, AMA 4720
WACO Brotherhood #1