Shiny is not the desired end product.
It's simply the result of using very fine particles in a paste to abrade the surface and clean it. You can also use fine wet automotive sandpaper, compounding paste, whatever. It's the same thing.
After cleaning the copper to remove the oxide, it's a lot easier to solder.
I think most people don't recommend steel wool to clean copper, but maybe that's before the toner goes on, not after. I hate the stuff, never use it anywhere. I use green scotchbrite pads and water instead.
I've made over 3 dozen toner boards this way, and it works great for me. I use a laminator to apply the heat, and Pulsar products, including the green film over the toner before etching. Also, post etch use toner and the Pulsar green film to apply the text / top parts designation ("silk screen") which makes it a lot easier loading parts.
Bob