All paint should be removed from contacting surfaces. Also, be careful not to damage any painting you have done. Different glues are available for different needs. Tube cements weld the plastic together in a permanent bond. Used sparingly, tube cement is very effective.
Liquid cement is applied while holding the parts together.
The entire seam is fused by allowing the liquid cement to run along the joint. Crazy glues are also a possible option. Glueing clear parts requires a steady hand or a white glue. If you have a steady hand go ahead and use cement. Otherwise, use a white glue that will dry clear but weaker.