Thanks for your replies. I did sand the areas pretty well, I think. This area on the frame was mostly surface rust, but I sanded it pretty thorougly, anyway. Some parts of that turned black in the seams, but the flat area didn't, and the red rust was still there after it dried a couple of days, and I sanded the white stuff off. Another area, a battery tray with some pretty healthy rust, I mostly scraped with a putty knife and sanded some, but a good bit of the coarse rust did turn black. I think I'll try some other, maybe solvent based, brand. Especially when I use it on a bike.