Typically, I use Perfect Plastic Putty or on occassion Tamiya Putty (the gray stuff) to fill gaps and seams. Fuselage halves, wing halves are no problem as to a degree the putty (or excess glue/plastic) will sit proud of the gap when it's sanded. Flat surfaces, however, are a different story. With either gap filler, I find you still see where the plastic ends and the filler begins; feathering is a bit of trial and error.
A couple of things I've learned recently: circular motion seems to feather the putty better and sanding from the putty out to the plastic also works a little too. Using a wet (or dipped in acetone) Q-tip helps, though you have to be careful not to overdo it or you're back to square one. I'm awaiting some Tamiya Surface Primer in the bottle to see how it works and if it will blend better. I've also airbrushed some Alclad Gray Primer & Microfiller, but it is very fine and seems to require many layers with little result.
This problem is multiplied in my current project which is pretty much all metallic finish which shows everything on the surface. Patience seems to be the only solution as I fill, sand, prime, repeat (not like I'm unique in this).
Anyone have any solutions or techniques? I've gone through about 50 pages on the forum but haven't found anything that specifically addresses this.
P.S.
Use very litle pressure on your sandpaper/sanding whatever: let the sander do the work.