Use a rubber head hammer or rubber head mallet, not your claw hammer to knock out the dent in aluminum.

Make sure you have your garden hose near by after giving it a few hits with the rubber hammer. Aluminum heats up and weakens when you bend it, like a empty soda pop can.

So squirt it with the garden hose if it feels warm or hot where you hit it. This keeps the aluminum cooled and hardened, or you might put a hole in it, banging it out while it is warm or hot.