Hi,
smear a thin film of oil, butter, or similar, that the sealant won't adhere to on one of the mating faces and make sure that the thickness of the sealant is kept fairly equal all way 'round.
1..3mm should do, depending on the hardness (shore) of the sealant.
Test first with some scrap.
Is it to be used in water or just to be protected from sprays?