Use of fixed sealant like fibreglass on a moving panel joint will break /crack so defeat its object.
I use flexible mastic, builders use for external work. white or brown or clear.
Use with mastic gun DON'T cut tip of nozzle lay it in three passes press in first pass lightly on second and third passes smooth with finger dipped in water.
You want
mastic, application gun, water bowl, rag/tissue, white spirit to clean area then wipe over with soapy water dry thoroughly dry. use hairdryer to quicken drying. Once dry make first pass when still tacky second pass same with third pass. USE wet finger on last pass to smooth down over the joint.
BOB UK, July 2014
