The C3 External corner is a hot press moulded GRP pre-formed external corner that removes the need to mitre corners of D260 fillet trim. The corner should be nailed to he deck at the corner and in the middle. The joint between the trim and the corner should be sealed with a thin line of PU sealant. Any excess sealant should be wiped clear. The joint should now be bandaged with 75mm wipe glassfibre mat.