[bigeasy1]

Several years ago a friend and I repainted two identical 18' Starcraft aluminum boats.I sprayed mine with Sherwin Williams(automotive) Sunfire which is a two part urethane, mostly used on airplanes and fleet trucks.
He sprayed his with Imron.

Both of them looked great and both paints were extremely durable. We went on a three week trip to lake Champlain and both of these boats were in the water for the whole time.
When we pulled them to head for home,his boat with the Imron paint had all these tiny tiny blisters(felt like mild sandpaper)below the water line.
My hull with the Sunfire was perfect.

We had no idea why the difference,but he called the automotive jobber and was told that they don't recommend Imron below the water line.I never checked on the sunfire,but after ten years of use,and abuse, and it looked as good as the day it was painted.

On my 23 Seacraft,I used Epifanes two part urethane on the hull,(only to the waterline),but I noticed in a couple areas where the paint sits a couple inches below the water, it also is getting some tiny blisters.The people at Epifanes told me that it's a saturated, aliphatic urethane which is similar to Imron.Maybe that's why it blistered when under the water.
I just figured that all two part urethanes were of the same chemical makeup,but was told by Epifanes that there are differences in the makeups of all of them.

I would assume that's why some paints like the Signature and Sunfire will hold up where others don't.