Well, first at all don't use any silicone based things with cork. With cork only clean properly both sides from head and cover. Don't touch greasy hands that cork. Put cork on water and touch it by hand all over in water. Then just put in on place and use correct moment. Same thing with oil pan seal. I know this sounds bit odd but I have learned this 25 years ago and it has worked nicely. I learned this from very old Peugeot mechanic (died 20 years ago) and he learned when he was studying it in peugeot factory in 50's
Funny thing is at some US based car factory manuals gives guidance at that sould be oiled (LoL) and that will only do at it will leak surely
V-M