Xtracolour paint made in the UK is gloss finish for the reason you mention, to avoid the necessity of overcoats. BAsed on what I've seen in SAM and SAMI magazines, it appears to give very good results. It is available from a few mail-order shops in the US. Also, you may find that even with flats, acrylic paints give a very smooth finish. Again, overcoats help to make the different colors of many camouflage schemes match in reflectance, giving a more even finish. For me, flat coats after gloss coats usually yield a semi-gloss or satin gloss finish, which is often the original value of the finish of the actual aircraft.