Hi monster, welcome to the forum.
Re your question, firstly ensure that the surfaces to be joined are paint free.
Use liquid poly (I use the humbrol brand), don't bother with the brush that comes with the jar, its far too thick.
Get a small paintbrush and keep this to one side for cement only.
Put the parts together, load your brush with glue, then touch the brush gently on the joint. Capillary action will draw the glue into the seam and leave your paintwork untouched.