Hi. I'm still pretty new to this hobby, only been building about 2 years. I have never had any sucess with the Squadron Putty, it seems to gritty and it piles more than i would like. I have used Mr. Surfacer, Tamiya Putty, and 3M Acryl Blue with fairly decent success. I have found that with Surfacer, one coat won't do it. I usually have to apply several coats (sometimes 3 or 4 coats depending on the seam to be filled.) Tamiya putty usually fills with one coat, but again, if the seam is wide and/or deep I have had to use multiple coats of that too. The Tamiya putty goes on "wetter", and i can manipulate it into the seam a little better. 3M Acryl blue isn;t as wet and Tamiya putty, but it too goes on smooth and cover fairly well.
The only advice I have, is to layer it on in thin coats, and slowly build it up.
Not sure how much help that is, but hopefully there is something you can use.
Take Care.
Brian