I fully operate well within the scope-creep range on all of my projects. Sometimes managing the scope-creep takes more effort than managing the project itself!
FWIW not all micro-spheres are created the same. Those intended to be used as thickening agents in resin are usually full of air and...
I always figured that if I went with bed liner inside that I'd first coat the inside with one of the moisture curing polyurethane paints (POR-15, Rust Bullet, etc). That would seal off that side of the metal from any moisture. Those familair with them will tell you that I-H Scouts are made from...