Painting pieces before gluing has some benefits but can also be very problematic, and usually is.
I'm sure you already know this, but first mask off the surface edges to be glued before painting. Or, of course, scraping them clean before gluing.
Making Repairs:
Most importantly,
always apply enough paint on the model so you can go back to sand any seams or surfaces where the glue may have seeped out a bit. If you're using flat paint, you can go back and make touch-ups with the same color.
(I'll assume you're
not using "tube glue" as it is much harder to repair)
For touch-ups; first sand the visible glueyness out of the seam as best you can. Then thin the (base color) paint a bit and carefully brush it into the glued seam. You can then micro sand it into the finish. You may have to repeat the process two or three times (or more) before it all looks right, but it will work.
If you notice that you have some unexpected seams in a finished surface after painting and assembly, you can fill them in by using the same method. Again, think ahead when spray painting. Apply enough paint for the task should the need arise.
To fill in tiny seams and gaps, you use the same base color. But this time you need to let the paint thicken a bit, so it becomes almost like a paste. You can do this by globbing some into a small cup and just letting it set for 30 minutes or so. Carefully brush it into the seam and let it dry. It'll probably shrink a bit so wait a day and then brush on a little more until you're sure it has filled the seam. (Repeat if nessa) Then do the same micro sanding as mentioned before.
When micro sanding a repaired section use 800 grit wet/dry paper with water. Use just the >lightest touch< on the painted surface when sanding.
A side note: I always do a final light sanding on all my paint jobs anyways to get rid of any "tooth" in the finish, so any repair sanding usually blends right in to the finish.
It takes patience and persistence, but most finishes and problem areas can be touched-up using this method. It works pretty good with flat paints. I haven't tried it with gloss finishes yet.
Hope this helps.
