I saw a neat trick where a guy used a piece of PVC conduit with a cap on one end to soak a hood prop. Long piece but using the conduit just big enough to slide the piece in saved him a lot of vinegar.
That might have been me. I also put a small ball valve on the bottom so I could drain it without making a big mess pouring it out the top. You can see the 2" PVC pipe zip tied to the leg of my work table in the attached pic. But I've been using Evaporust.
[on edit] Another trick for a flat piece is to lay some lumber on the floor and then some heavy plastic sheet (I save stuff like old shower curtains) to make a flat basin that will just cover the piece (like the transmission cover). I've done that on similar items.
Another trick for the floor would be to take an old terry towel, soak it in vinegar (or your favorite derusting solution) and lay it over the floor, keeping it wet for several days.
On the fiberglass trick, if there are pinholes, that helps keep it bonded to the floor by "keying" it in. But the metal should be relatively rust-free. Also, instead of polyester resin, you can use epoxy resin with the fiberglass cloth.
