Seam Filling

I have found that Mr. Surfacer 500 works great and is easy to sand smooth after it dries.  It does shrink a bit so you may have to use two applications.

Also Vallejo Plastic Putty works well.

I have occasionally used white glue to fill a crack or seam. It is like the caulk. You cannot sand it, so I only use that where I am absolutely sure that I will not need to sand!

I also find the full-bodied primers I use (usually Krylon, sometimes Duplicolor or other auto body primers) can also fill minor cracks, seams or depressions. On larger gaps I use an auto body filler (now called glazing putty, for reasons I don't understand- it used to be called "spot putty").

I have heard some use latex caulk effectively but it tends to be tricky to sand, so I guess it depends on where it is used.  I personally use just good old Testors putty or Green putty, let dry and CAREFULLY sand.

I use Bondo for a lot of seam filling.  Not the two part stuff, but the one part stuff that comes in a tube like toothpaste.  For bigger jobs I use a product called Apoxy-Sculpt, which is a two part epoxy like compound that once mixed can be rolled into a snake and fitted into a seam.  Sands well once dry.

Bob

An easy solution is mixing glue and sprue into a goo, use a toothpick to apply gingerly in gaps. Give it a healthy amount of time to cure and dry before sanding. I'm also fond of lightly sanding, so a gap fills with dust, I clean around the edges then seal the dust in the gap with a fine line of super glue. I avoid using super glue to fill though, I personally don't like sanding super glue. Less is more basically.

I've yet to find a putty I like, the next thing I want to try is wood filler... god only knows if it'll work.

Ive found that a whole array will get the seams filled, with various results and warnings. 1. I find that regular model glue in a tube is just fine for a lot of small seam work.  Yes, it MAY craze the plastic if not wiped smooth, but you're going to sand it afterwards anyway.  2. I only use seam filler Super Glue type cements after Ive used something else.  I find that even those labeled "SEAM FILLING" are too watery and need several coats. 3. White glue like Elmer's works very nicely and leaves no marks.  4. Wood glue is thicker than white glue and works very well for bigger gaps. 5. For small seams, stretched sprue is good but takes some effort to get right. 6. The problem with model putty is that altho it can be feathered, I find it tends to always have some leftover shreds that aren't easy to handle afterwards.  More important, the damn stuff dries out in the tube very fast once opened. It is a real pain to buy a tube and use so little--as with Super Glue very often. Better and cheaper is good old Spackle. The fact is, once you stick come gap-filling stuff in here, sand it and paint it, it will look just fine. 

This should help you a great deal  PERFECT SEAMS . If you build using the proper methods and techniques seams shouldn't be an issue. However, when they do this link should help you do it better.

FSM article stated that you can use Mr Surfacer 500 to fill cracks,just paint it into the gap and instead of sanding,it can be smoothed out by a swab with alcohol until it is smoth.Have yet to try it,but it sounds good.

Sadly not really. To fill seams, you need a material that can fill and feather out when sanded. 

What did you use to build the model? If you've got some thick, gap-filling super glue, you can use that to fill seams in lieu of putty.