Putty

I would use Mr Surfacer 500

I too must 2nd Mr. Surfacer. If you don't like getting your fingers messy and covered in chemicals you can use Q-tips in lieu. If you don't have lacquer thinner a cheaper alternative you might like to consider is Isopropyl alcohol. Works great.

Once in a while, I come across a very fine seam that has to be filled. I use White-Out in the pen type applicator. "Draw" the line with the tip and the white-out flows. Easy to wipe off any excess. 

Jim 

I have indeed used white glue for very slight seam openings.  You must apply it perfectly- white glue cannot be sanded, so it must nestle down into the seam.  It then can be painted with normal paints and methods.

in addition to what everyone else said put tape as close to the seam as possible on both sides, add putty pull off tape while the putty is still soft and knock down the ridge with solvent (i use 3m red bondo and acetone/nail polish remover). you can retape on eithet dife before sanding and keep replacing the "protective " tape as it wears.

a while back someone said they use the expanding gorilla glue for wing roots. it foams up as it dries leaving a ridge that can be scraped off.

if the gap isn't huge white glue might work too. i use it a lot for small seams in hard to reach places.

Sounds like a job for Gunze "Mr. Surfacer."  It's a thick liquid that you can apply iinto the seam with a small paintbrush, let it dry, then reapply as necessary to fill the gap.  You can smooth it out with lacquer thinner on your fingertip.  It works very well.

For kits with recessed panel lines it is fairly easy to rescribe the lines in the area of the putty/filler.  For raised panel lines and other detail it is a lot more work, but it is what you need to do.  Try to use an application tool so that you put on as little putty as will do the job, and scrape any excess away before it hardens.  Many putties can be thinned with lacquer thinner.  Some putties say on tube what they can be thinned with.  Thinning may make it easier to get just where you want it, with a brush or other tool.  It is best to thin the putty rather than do wipe it with a solvent.  Again, if you DO get excess on plastic, scrape with a tool.