Squadron Green putty
(http://www.squadron.com)

Squadron Green Putty has been used by years for modelers. I use it to fill my seams and smooth surface areas. I use Acetone (Cutex finger nail polish remove) to thin and smooth down the putty. Basically you thing the putty down and use a knife or a brush to fill in the seam and wait for it to dry. If you thin it you will want to make sure the putty is thick as it will shrink more than it usually does. I then sand the putty down fairly well and then use a Q-tip dipped in acetone to smooth and remove excess putty off of the model.

They also make a white putty that supposed to act the same as the blue putty but the whiteness will show though when you prime it.

Contact me at josborn@happinessismandatory.com