A coat of mineral oil immediately after soaking helps, but a thin coat of petroleum jelly blocks evaporation much better and it isn't toxic or any bother to the iguana at all. It only has a yuck factor with the humans.
Chronically stuck spike sheds are a tough problem. After tubbing,, the spikes need a thicker application of Vaseline. Buildups of spike sheds strangulate the spikes and they are damaged or killed. Dead ones fall off, but I've never heard of infections coming of that. I tried the fingernail technique for peeling spike sheds for a while, and the spikes would turn red from internal bleeding. I then started to find the edges (it takes good eyes or a magnifying glass to see them) of the sheds at the base of the spikes and pulled up on them to peel them off a strip at a time, but I never was sure it didn't do unseen damage. This iguana ended up with ragged, split, and bent spikes.