yes, with liquid mask it's difficult to get a straight line. try this, cut thin strips of masking tape, apply along the side of the frames (easy on a 109) then fill in the 'holes' with the liquid mask. As was said above, Tamiya masking tape is very good, it's what I use. Yes, it is fiddly to apply, but if properly smoothed down it will do it's job, peel off nicely and not leave a deposit.