2 hrs is pretty good for it to skin over and yes it does need some cure time before filling. I didn't realize that so much until I let a Plymouth dealer replace my axle seals in warranty. Something made me suspicious, so I checked to see how they tightened the bearing nuts. They were as tight as the guy could get them! Then I got suspicious as to why they had the diff cover off. I pulled the cover and found everything inside covered with silicon sealer. The bead of excess sealer around the cover washed off with the lube. The guy must have filled it immediately! I took a garden sprayer and a gallon of lacquer thinner to clean it all off. It wouldn't have been so bad if he had'nt of used so much. There was a 1/4" bead around the outside(that's why I suspected sloppy work) that he didn't clean off so I suspect there was that much inside too. Craig