I've always done a 12" square pad of 3/4" ply under each
leg when on grass, haven't had a problem with anything sinking yet. Take plenty of spare pads, including some of other material thicknesses so that you can use them as shims to
level the
stage - grass areas are rarely a true smooth and
level surface. I've had to deal with 10" of fall across a
stage not much bigger than yours. Doing a little reconnaissance with a
laser level before
load-in can be a good idea.
Usually a 1x4 brace from the back of the flats down to a 2x4
cleat screwed into the
deck is sufficient as a wind brace, going out about half as far as they go up (roughly a 30-60-90 triangle), and putting a brace at each
flat seam, roughly 4' apart. You might want to extend the
deck upstage of the
flat line far enough to accommodate the bracing. It depends on your situation, though - down in a valley surrounded by trees and buildings will see less wind than atop an open hill or ridge.
I've always used exterior latex paint front and back to seal the flats (my shows were two week runs), and that seemed to be fine. Exposed wood will suck in a lot of moisture and warp if you get rained on, better to spend a little more time painting in the shop than a lot of time fixing on site. Painting the undersides of the platforms is less important, unless they're
flat on the
ground, with no legs.
Make sure to check in
advance if the site owner is fine with the
platform legs or overall
deck killing the grass. Even just laying a
flat on the grass for a few minutes on a hot summer's day can be enough to
shock the grass dormant, I've learned to keep things on the truck until they're ready to install.
