Reading that it forms a "flexible waterproof skin" I would have concerns over how thick this stuff is. The stem/steerer tube interface is pretty tight fitting and unless it is completely cured like a paint, I could see it having durability issues when you try to slide the stem on. I think I would just install the stem dry and get it all tightened down and then apply a thin film of waterproof grease with your finger to any areas that might be in contact with the elements or let water in. I am not a fan of grease on this area as it requires additional torque to stop the stem from slipping, but if you do, make sure it is very thin, your pinch bolts have enough torque you should be ok—given the fact that we are talking about a steel steer tube.
Alternatively, when I had one of my frames painted, the entire steerer got a very thin coat of primer, and the rest of the fork was painted with multiple coats of paint. The primer is thin enough that it doesn't affect the OD of the steerer and keeps it completely protected from moisture (this is especially handy since I am using a Thomson stem which has windows that expose the steer tube on each side).