I use Swannys "Fencing" method. You simply border the seam with tape, apply putty, remove tape, swipe down the putty with a qtip soaked in acetone based fingernail polish remover, then sand.
I will say one thing though, depending on the type of putty you use, some will shrink when they fully cure. So a fully filled and sanded seam can contract and youll have a seam line. So I find its best for me (Im using Squadron white putty) to do the fencing method then wait a couple days before I sand.
Almost every other time I did the fencing thing I had other stuff to do and couldnt get to the model for a couple days anyway. Recently I had a P-51 and too much time on my hands. I puttied and then sanded almost immediately afterward. After I painted it I found a nice filled in seam line! I kinda screwed the paint up anyway so I decided to strip it all using Castrol Super Clean. Right now Im back to the stage where Im getting ready to putty that same ol seam... this time it wont come back!
Fighter pilots make movies.
Bomber pilots make history.