Measure the length and width of the existing tabletop with a tape measure. Take the measurement several times to ensure accuracy.
Purchase a 3- or 4-gauge sheet of galvanized steel. Have the sheet cut to the length and width of the table. To ensure an accurate cut, have the professionals at the home improvement store where you purchase the sheet cut it.
Sand the edges of the cut galvanized steel with a fine metal sander to smooth it out.
Set the sheet of galvanized steel on the surface of the tabletop so that the edges of the sheet match up with the edges of the tabletop.
Drill 1-inch flat-head metal screws through the surface of the metal into the tabletop to secure it in place. Set the screws so that they are 1/2 inch from the edges of the table, and secure a screw every 4 to 6 inches around the edges of the table.