Between the cold night and laying on the Ospho a little thick, I ended up with a lot of the powder byproduct. Well, that and I was applying it to galvanized metal. In a couple of dents, the product pooled and it is not yet fully dry. So today will not be a day of painting.
I worked a ways using a scrub brush, but I’ve decided to get a new brush attachment for the drill to clean up this mess later today. The rust has indeed turned black, though, and galvanized parts have been etched nicely. All in all, it has been a success. An annoying success.