This section is from the book "Soldering For Workshop, Farm And Home", by John Bonert. Also available from Amazon: Soldering For Workshop, Farm And Home - Information On Soft And Hard Soldering - Projects For The Workshop Explained And Illustrated.
The next practice will be soldering up a hole in a piece of sheet metal. If the hole is very small it can sometimes be covered by tinning but if the hole is large a patch will have to be put over it. If an attempt is made to solder up a hole the solder will run through and it will not close the hole up. In order to close this hole a patch will have to be put on.
To try this a hole can be punched in the bottom of a tin can. Drive a center punch or a large nail through the bottom to make a hole about 3/16" in diameter. Clean the surface for a distance of about around this hole. Cut a piece of the metal from the same can, or another one may be used. This piece is cut larger than is needed to make handling easier while tinning. Clean a spot about 3/4" in diameter on both sides of the metal. Tin this spot on both sides. After both sides have been tinned cut a circular piece out of this tinned spot about in diameter. This piece will be used as the patch to cover the hole. Next, tin the surface around the hole. Both the surface around the hole and the patch are now ready to be joined. Apply a little rosin or tallow candle on the tinned surfaces and place the patch over the hole centering it as near as possible. Use a pointed stick of wood or the point of a scriber to hold the patch in place. Now place a flat side of the point of the copper on the patch and hold it there until the tinning on the can and patch melts and fuses together. It may be necessary to add a little more solder to cover the patch. Remove the copper but hold the patch down until it cools.
If a hole is to be soldered in a cooking utensil coated with agate or enamel, the agate or enamel will have to be removed by chipping or grinding so that the metal around the hole can be cleaned for tinning. Putting on a patch in this manner is called sweating. For any work where two surfaces are joined in this manner the operation is called "sweating." This sweating will be used again for some of the operations to come later. If a large hole is to be covered the same patch method may be used but of course if the hole should be an inch in diameter or larger the sweating cannot be done by merely holding the copper on the patch. In such a case the copper will have to be moved all around the edge of the patch to distribute the heat.