The paint does work, but probably wouldn’t adhere well to the back of a plastic pickguard, and is probably better for smaller cavities. For a Strat I’d be more inclined to use copper foil. Whatever you use, make sure that all areas of shielding are connected to each other, and to the ground.
I'm probably going to get an aluminium shield plate to go under the pickguard. Looks like considerably less faffing than tape (same goes for the paint, tbh).
The paint does work, but probably wouldn’t adhere well to the back of a plastic pickguard, and is probably better for smaller cavities. For a Strat I’d be more inclined to use copper foil. Whatever you use, make sure that all areas of shielding are connected to each other, and to the ground.