While putting together my latest 1/350 ship, I discovered that I need to use alot of some type of filler around the bulkhead to deck assemblies. TESTORS RED PUTTY works great for this. Take a small paint brush, like maybe 00, dip it in some LAQUER THINNER, then pick-up some putty with it, diluting the putty, making it real thin. You can then litterally brush the putty in the cracks you need to fill. This works very well with most any size gap. Just pour a little putty out, and mix in some LAQUER thinner with it to proper consistency, for whatever application you need it for, and paint it on. You may need to do a few coats to properly fill in the gap, depending on the size. Be kind of carefull with the amount of LAQUER thinner you use because to much might attack the plastic.
(below) Here im applying a small amount of putty with a 00 paint brush.
(below) Here im dipping a small amount of putty straight out of the tube. This way is much easier and less messy, and works just fine.
(below) I use Testors Red Putty and just regular Laquer Thinner to thin it.