Work the original rubber gasket from the antique refrigerator door. Look for a seam. Some antique gaskets were installed in a rubber loop but some have an overlapping seam. If there's no seam, begin at the top of the door and work your way around and down. There may be screws or metal clips holding the gasket in place. Some gaskets fit inside of a slight indention around the door. If there are screws present, do not remove them. Loosen the screws enough to allow the gasket to be pulled free.
Fill a bucket with hot water and baking soda (1 cup baking soda per gallon of water). Clean the area beneath the gasket with a sponge. Debris left where the new gasket will be installed can compromise the airtight seal. Thoroughly dry the area with a dishtowel.
Measure the width of the original gasket. Measure the length (or diameter if a continuous loop) of the gasket. Because the appliance is an antique, the manufacturer may not exist anymore. A replacement gasket can be used. Locate a new gasket from online restoration and appliance repair stores or from appliance supply stores. Choose a gasket a little thicker than the one removed. Time has thinned the original gasket, making it appear thinner than when installed.
Place the new gasket into a bathtub of warm water until the water cools. This warms the rubber, making it easier to install. Dry the gasket with a dishtowel until completely dry.
Attach the gasket to the door, starting at the center of the door's hinged side. Depending on the model of antique refrigerator, slide the gasket over or under any metal clips. If the gasket attaches over or under screws, use a screwdriver to tighten the screws enough to hold the gasket in place, but don't completely tighten the screws until the gasket is installed.
Cut any excess rubber gasket if the replacement gasket is not a loop. Rubber gaskets are sold in foot increments and pre-packaged measurements. Once completely installed, tighten any screws completely. If there is a blemish in the gasket prohibiting installation, use a hair dryer to warm the rubber, pressing the gasket into place. Hold the hair dryer 3 inches from the rubber.
Close the door. Try to slip a dollar through the closed door. If the dollar won't slide through, your gasket has a good fit. If there are gaps, order a thicker gasket.
Lubricate the gasket along the door's hinge side annually with petroleum jelly or parafin wax. The gasket along this side of the door is the hardest working part of the seal and is prone to twisting and damage. Lubrication keeps the rubber flexible.
Save the original gasket (or a portion of it) for future reference and for future replacements for your antique refrigerator door. Save the replacement gasket's information, such as the receipt and brand name.