I usually test fit all my parts before gluing and painting.

When dealing with seams and or gaps, I'll sand it down lightly to a smooth finish - especially when glue seeps out. Sometimes, I'll use the blade of the hobby knife, then go over it with a sanding stick - preferably a light grit.

Even if you still see sort of a seam, sometimes I'll use Mr. surfacer and dab with a brush to fill it in. Sand lightly once dried sufficiently

As for gaps, there are different methods. My go-to is Perfect Plastic Putty by Deluxe. Easy to wipe down excess using water.

Another method is using small sections of sheet styrene then fill in with putty.

Another I have seen folks use is homemade Sprue glue. Do a search on YouTube on how to make your own sprue glue. I've never done it but it's something I thought about trying and experiment with it on a mule kit in the future.