Thew reason paint shave the finish they do is because of the surface of the cured/dried paint. Gloss will be extra smooth so much off the light bounces back towards your eyes. Flat is going to be very porous so the light get refracted in many directions.If you use a gloss clear, you will achieve a much smoother end finish than with the flat paint only. It will be very shiny in comparison. If you want a flat finish, but protected by clear, you need clear that has the finish you desire. Getting matt or semi-gloss is available in spray cans. I haven't seen a true flat in a can, but maybe. You can spray a very flat clear with spray equipment though, but I'm guessing that isn't a possibility by your initial question.