What's the best way to protect metal siding when pouring a wall?

The lime in that concrete will absolutely eat through the coating on your corrugated siding if you don't get it cleaned off right now. Wet concrete is highly alkaline and triggers **galvanic corrosion** on galvanized steel or pitting on aluminum. If you've got splash-back, grab a bucket of clean water and a sponge while it's still wet. Once that stuff cures, you'll be scraping it off and ruining the finish on the shop building. For a low wall, pulling the forms after 24 hours is the standard move. The concrete is still "green" at that point, wich is acutally an advantage if you want a clean look. You can use a damp sponge or a masonry brush to smooth out any honeycombing or lines left by the wooden forms. If you wait three or four days to see the finished face, those marks are permanent and you'll have to grind them out.

It sounds like you're doing the manual labor the hard way, but don't let the fatigue convince you to add more water to the mix. Poured-in-place walls that go in like soup always end up with more shrinkage cracks. If it's alredy in and you're just smoothing it, keep at it until the bleed water disappears. Adding more water now just to make it easier to trowel will compromise the strength of the surface. Since you're worried about the sun, once it's firm enough that you can't push a finger into it, you might want to mist it down or cover it with some plastic. It prevents the top from drying out and "burning" before the core has a chance to set, especially with that metal siding reflecting heat right back onto the wet mix. If it dries too fast, you'll get those tiny spiderweb cracks all over the top edge.

Man, that back-breaking part is no joke. I tried a small footer last month and my lower back was shot for a week. I didn't realize the metal siding coud acutally get ruined that fast though.

Is there a specific trick to getting the plastic to stay put against the metal if the wind picks up? I'm planning a project next to my shed and I'm worried about the sun reflecting off the siding. Also, does it matter what kind of wood you use for the forms? I have some old plywood scraps but I'm worried they will stick too much.

For the wind issue, I’ve had luck using magnetic clips or even some painters tape to secure the plastic right to the metal siding. Just don't use heavy-duty duct tape or you'll be dealing with adhesive residue for months. If you're still pouring, try tucking a strip of roofing felt or even some thin cardboard between your forms and the metal. It acts as a sacrificial barrier against the splash-back. Once the concrete is set, you just pull the cardboard out. It’s way easier than scrubbing lime off galvanized steel after a long day of troweling.