Mold lines should sand off and not leave a "ghost". If they do it's due to a manufacturing fault (it does happen) and you'll have to use some kind of putty or surfacer ,followed by more sanding,to get rid of them.
Assembly lines,seams,are a whole other thing. Plastic cements "weld" parts together chemically by dissolving the plastic of the mating surfaces with a solvent. The first thing is to make sure that the mating surfaces are infact mating. Warping,bad molding or something as simple as a badly molded locating pin can sometimes prevent this.
Everyone uses different glues but once you are happy that the two surfaces to be joined mate perfectly you should apply the glue and tape or clamp the parts together. Hopefully you should get an ooze of "molten" plastic from the seam. Leave it alone until dry. When you scrape this dried plastic off,the seam behind it should be filled (with plastic) and not need any further filling.
Someone did a very good article on seams on another site and if I can find it I'll add a link to this post.
Cheers
Steve
That was easy! Everyone should read this,however experienced. Gerald Voight has produced a superb tutorial in this thread.
http://www.aeroscale.co.uk/modules.php?op=modload&name=SquawkBox&file=index&req=viewtopic&topic_id=111677&page=1
Steve