Resins: Isophthalic vs. Orthophthalic

Just below Nellie's bootstripe the hull has been sanded to reveal (starting in the pictures upper right and moving to the lower left) multiple layers of bottom paint (black and red), gel coat (bright white), barrier coat (gray), and fiberglass.

Tommy Chen said that he used two types of polyester resin (at least on the later tugs). Below the waterline he used an "iso" resin as it resists osmotic blistering.  Above the waterline he used "otho" resin.

"Orthophthalic polyester resin is the standard economic resin used by many people.  Isophthalic polyester resin is now becoming the preferred material in industries such as marine where its superior water resistance is desirable." From Net Composites.

To decrease the permeability below the water line iso resin was mixed with what Tommy called a "gray powder" and then applied to the hull with a roller.  I assume the gray powder contained plateletes, like those in Interlux 2000E, which make it more difficult for water molecules to pass through.  So effective is this gray layer of iso that Tommy says he made some tugs without gel coat bottoms.  Nellie D. #63 clearly has the gray layer between the gel coat and fiberglass.  According to Tommy, the bottom's gel coat should be about .7mm thick.

I measured a piece of Nellie's gel coat, which has the gray layer on it, at .027" or .6858mm.  The gray layer is very thin.

Dave Howell
Nellie D. 37VT63