You really need to get loads of the sealant behind the outside seal and bodywork. You just keep on pumping until your hands ache. It does take a surprising amout of sealant to fill the all the gaps. You need a finish as below in the picture, so the rain just runs off and none of it goes behind the seal. It just forms a small lip of sealant. If you have done it right the outside of the seal should be bulging and full of sealant that stays pliable.