I found a hand plane at the thrift store. I think this is a #4 smoothing plane, which I hope to use for finishing. But before that, it's quite clear that I'm going to have to clean this up a bit by removing the rust and making it look a bit prettier.
Any advice y'all have for how I might start restoring this?
Start with a dead-flat surface, like a surface plate or pane of glass, and some high-grit abrasive papers, at least 220. Disassemble everything, including to take the frog out. Run the sole over sandpaper on the flat surface to see if it's flat; back down to lower grits if it's really bad. Do the same with the machined surfaces of the frog.
Simple rust removal with a wire wheel, steel wool, naval jelly/Evapo-rust. The flat-sandpaper method is not ideal for removing rust from the lever cap and blade, because they're flexible enough that pressure distribution can change the metal removal pattern.