When I build a structure kit, every once in a while i get a situation where there is a fine separation gap between styrene parts, perhaps on the order of 1/64th of an inch or so. Sometimes, the painting process does not provide sufficient “filler” to eliminate this gap. Squadron putty seems a but clumsy to use here, particularly where the gap occurs in corners between flat surfaces, so if anyone has a good idea, I’m “all ears” !
You may try to dress up the parts by using some real fine sand paper on a flat surface before gluing. Clamping the pieces will help. I use rubber bands and mini bar clamps. I also use a thin glue like tenax and let capillary action draw it in the joints.
I’ve had similar problems and have tried Elmer’s white glue, squirted in from the inside or outside with the opposite side covered with firmly taped wax paper. It may take several appilcations after complete drying to “build up” enough thickness so the edge can be painted, and since the glue is so liquid, you"ll need to balance the structure properly for it to dry in the crack and not run out.
The other technique is to use the Squadron putty but I agree that it is difficult to get it exactly where you want it, so I tape the wax paper along the outside seam and just force the putty into the crack from the inside with a q-tip. This can be sanded or filed where the Elmer’s really can’t.
With styrene you have a few options. One way would be to fill the gap with super glue and sand, or an even better way is to thin putty (like squadron white) with plastic cement and use this mixture to glue the pieces together to begin with. Squeezing together the parts will cause the mixture to ooze out…just wipe away the excess after the gap is filled and sand after its dry. Another way would be to mask off the gap with tape, sparingly apply putty, wait for the putty to set a little bit and then smooth it out with acetone fingernail polish on a q tip and then sand when dry. All 3 of these ways should get rid of the gap with no evidence that it was ever there. Good luck!
I would first assure that I had done everything I can to not have the cracks in the first place. But even so, there are situations where they just “happen”.
One thing you can do is use real caulk - the kind of painters caulk (not silicone) we use for adhering track and cork roadbed. Make sure the label says “paintable”!
Get a dab on your finger and rub it in and smooth it off, then let dry for a few hours. Usually you can get it just right with only using your fingers, but sometimes a small metal painters spatula will be a help.
I have used Testors glue in the tube to fill gaps in styrene. It is fairly easy to get into place and will be firm enough that it won’t run out in 10 - 15 minutes. The one downside is that it can disolve or reshape delicate parts. I made the mistake of using it to fill small gaps between Grandt Line roundhouse windows and the surrounding .040 styrene walls. The glue caused the window frames to swell slightly with the result that some of the muntins were deformed. For filling a gap between two pieces of relatively thick styrene it works quite well.