[Gerry Zagorski]

If it has some growth and or slime on it you will want to get all that stuff off before painting and yes, you can use your bottom paint.

I would hit it with some on/off to loosen things up and scrape or sand lightly if needed being careful not to damage the paint on the bracket. If you have areas of the braket that are down to the bare metal you want to spot prime it with the special primer used for aluminum or your paint is going to eventually bubble.

In order for the bottom paint to stick you need a good semi rough surface to adhere to so a light sanding will do just that. Make sure you wipe down the area after sanding to be sure you get all the dust off before painting it.