Galvanized Vent Hole Plugs, whether Aluminum or Zinc, will seal themselves very tightly with normal installation. The slight taper in the body allows the plug to be driven into the hole like a wedge. Each successive hammer strike drives the plug in slightly further and makes it a little tighter. As well, both Aluminum and Zinc Vent Hole plugs are relatively soft, malleable metals. While being driven in, they conform to the minor irregularities of the hole. In this way with normal installation, the plug is sufficiently tight to remain in place for the life of the fabricated steel.
Steel Manufacturing Blog: Keeping it Steel
Here's a quick note regarding Vent Hole Plugs. The large majority of customer requests are for aluminum Vent Hole Plugs. However, they are also available in zinc. There are several reasons a fabricator will request zinc rather than the less expensive and more malleable aluminum. The foremost reason would be potential corrosion. Mind you, we would be talking about corrosion over a very long period, possibly 20 to 30 years... but some jobs will call for the plug to be zinc.
Closing or sealing vent holes in any steel that has been Hot Dip Galvanized can be time consuming and expensive. If the closure is done by welding the Vent Hole shut, the welding process will also do a lot of damage to the galvanized surface.