Bathroom sealant PM sent

Bathroom sealant will certainly work but don't get it near anything you need to paint! An alternative is to dissolve some bits of scrap sprue into the liquid plastic cement to make a thin paste which can be brushed over the joints. This will help strengthen the bond as well as sealing any cracks. On my partially completed QM2 kit I just cemented a piece of plastic strip over the joint and pressed it into place.

Colin

If it is the left and right halves, with a long join the length of the hull down the middle, a fillet of polystyrene cement along the length of the join after they have been tacked into place will last as long as the plastic and be just as waterproof. Since it works by dissolving the plastic, allowing it to run together and then drying solid, effectively the two halves become one piece of waterproof plastic. A close mechanical fit is obviously a good thing. You can always add some extra strength (more for confidence than any real need) by sticking a strip of styrene sheet over the gap. From the inside, of course.

If it is a join between two parts that might at some time need separating, sealant by all means, but as stated above, it must not come into contact with any areas that will be painted, because they won't stay painted.