How do I make shiny galvanized steel look dull like the watering can?

Time and weather will take care of it. Pretty quickly ....

Hello Donna, I would spray paint it with Rustoleum with a "hammered" finish. You can purchase it at Menard's or Home Depot or Lowe's. Hope this helps you. Good luck.

It's a process , but it can be done

http://thegraphicsfairy.com/how-to-age-galvanized-metal/

Here's how I did it.....unconventional maybe but it worked. you will need toilet bowl cleaner, steel wool, rubber gloves and a hose. I put the pail in a large tub. squirt the toilet bowl cleaner liberally over the item and scrub down good with the steel wool, Let it set for about 20-30 minutes check if it has the desired patina .. Each piece you do will turn out different because of the manufacturing process. Rinse very thoroughly,,

Have you tried an acid ... like vinegar?

I throw my metal in my fire pit and enjoy a nice fire. Once the fire is out the steel will look weathered.

rub it with vinegar.

Scuff it with fine sandpaper and then spray it with vinegar or wrap it in paper towels soaked with vinegar.

Easy off oven cleaner in the blue can.

Try vinegar.