If I had to reseal I'd look at epoxy-based solutions, since that's what they used to mate the IHS to the PCB in the first place. Trick is to get the layer thinner than what comes from the packaging plant.
Which is why I just leave a small amount of the default epoxy on the PCB and clamp the HSF down to do the resealing trick. It's a very weak seal, but it's enough to hold the IHS on there when installing the HSF.
Some may not like that because of the air gap under the IHS that might still be there. But if you slice most of the stuff off and leave only a thin epoxy layer, it should be fine.