Polyester primer is a great product for things like pitted panels as you are working with.
"Slick Sand" is made by Evercoat,it may be sold by Eastwood, but it is an Evercoat product
(click here) .
I don't know of an Evercoat product called "Fiberfill" (they do have a new reinforced filler called "FiberTech") but they do make one called "FeatherFill" and is a polyester primer, not an epoxy. If it used MEKP, it is a polyester. In fact, FeatherFill and Slick Sand along with a couple others are in their polyester primer line.
The newest being FeatherFill "G2" (Generation II) and is a much better product than the old one.
There is no reason to worry about the flex of the hood. Polyester primer holds up real well.
Brian