re: wood butcher
The foam around my tank was saturated with water and VERY heavy because a pvc pipe under the tank to transfer water through the tank cavity was cracked and possibly blocked causing foam to eventually become saturated and cause pitting of aluminum tank.I dont think wet foam can harm a poly tank but I would recomend making sure water is able to freely pass through under tank to the bilge. I personally am not going to foam my new tank in especially under the tank.
Closed cell foam can repel water for days even months but will break down over time and obsorb water.
Ray