Yes, I painted the interior. To do this, I had to mask off the windows. I did this by putting a wide piece of masking tape over the outside of the rear window on the right hand side, cut it out using the recessed lines, then transferred this carefully to the inside left. I repeated, only vis versa for the other side. I covered the main side windows all as one- the framing of these windows shows through well enough even though I painted only the outside. BTW, I painted these seperately after all the rest of the painting, with strips of masking tape and hand painted the framing. Not a hard job with the recessed lines molded in.
I painted the interior before I glued the fuselage sides together. There is a short section of seam showing, between the rear piece of the floor I made and the rear bulkhead. I scratch-built the floor, as the seats in the J-3 do not side on the bottom, they sit on a plywood floor piece. BTW, the seat framing is incorrect and will not sit properly even on the bottom as you are supposed to build it. Either with or without a floor, you need to redo the seat "legs" to sit properly.