Me and my AE86

[Mux213]

Jezza, just the boot floor? I did the intire engine compartment of my RX7 with a sixpack (6 small cans, really the best deal because POR15 isn't very easy to store once a can has been opened, the paint that sticks to the lid causes the lid to fuse shut, and air left inside a can will ruin the paint).
(actually a starter kit + 1 or 2 extra of those small cans would probably do the whole boot)

As for metal ready, POR15 was sold for years and used for years without metal ready. Metal ready just etches the surface making POR15 stick better (or any paint for that matter). Especially on smooth new metal this is pretty important as POR doesn't stick perfectly to those surfaces. I would use it just to be safe, it doesnt cost that much, just get the smallest container they sell, its amazing how big a surface you can do with just a little metal ready.

Just wirewheel your boot floor where the rust has done its worse to get all the rust away, this will also help you find places where the rust has gone right through, you really should weld a new bit of steel in those places. Then put a nice coat of POR15 over it. This stuff dries pretty quickly which is also a good reason to get a sixpack. Within the hour you should put the second coat on (I never did btw)

After its dried for a day or so you can spray on a topcoat. Just sand it very very lightly and then use your run of the mill rattle can for something like the boot floor works fine for me.