I know of three classes of "waterproofing" compounds which have been used with varying rates of success. The first class consists of products which use silicone or a fluoride compound to increase the water repellence of the cloth. Basically an industrial strength "Scotch-Gard," they work by depositing a layer of moisture-repelling compound which prevents the moisture from getting a grip on the cloth, rather than by filling the pores in the fabric. Its downside is that it doesn't do as good a job of waterproofing as the other compounds and requires re-treatment about once a year.
There are several products of this sort. The most convenient,and most expensive, is a product called "Camp Dry" which is made by Kiwi (yes, the shoe polish people) and distributed through chains like Kmart. It comes in an aerosol can and covers, if memory serves, about 200-250 square feet per can. It has a distinct kerosene-like odor when applied, which mostly disappears with use. Since it doesn't saturate the threads of the fabric, it probably doesn't do much damage to the fire-retardant properties of the cloth or its ability to "breathe" (that is, allow water vapor to pass through the cloth).
Similar stuff, but water-based instead of solvent-based, can be had from:
The second compound is Can Vac, available from Panther Primitives, some awning supply outfits and possibly other sporting goods stores. It comes in liquid form, which you either brush on or spray on with a spray rig. It does provide a good watertight seal for canvas, but has drawbacks of its own:
One of the reasons people have had varying results in their use of Water Seal may rest in the product's formulation. The story I've heard is that at some point, Thompson's reformulated their product for the California market to comply with that state's "clean air" requirements. The formulation did significantly reduce the amount of noxious gases released into the atmosphere, but also reduced the effectiveness of the compound as a fabric sealer. After a while, the story goes, they've started withdrawing the old formulation from other markets as well, so different people in different parts of the country are liable to get the new formulation instead of the old one.
In a book entitled Let's Go Camping: A Guide to Outdoor Living by Harry Zarchy (Knopf, New York, 1951), I found this recipe:
"Dissolve a pound of laundry soap in two gallons of water. This can be done easily by first shaving the soap, then dropping it into the hot water and stirring it until it is completely dissolved. Soak the tent in this mixture until every fiber is completely saturated. Then squeeze out some of the surplus water, and hang it up to dry. If it's a large tent, you may spread it out in the sun.
"When it is thoroughly dry, prepare the second bath by dissolving a half pound of alum in two gallons of hot water. Immerse the tent in this solution and again saturate it thoroughly. It's best to leave it in this bath for a couple of hours. Then squeeze out the surplus and hang it up to dry again. Your tent is now ready for use."
I haven't tried this recipe myself, but it was printed in a real book with real ink, so maybe it works. Or, at least, it shows what people a half-century ago were willing to do to get a waterproof tent. I might add that this treatment probably leaves the canvas a bit more "breatheable" than solutions like Can Vac.
Another formula, found in G.B Colby and Bradford Angier's The Art and Science of Taking to the Woods (Stackpole Books, Harrisburg, Pa., 1970), is not for the faint of heart, or for those who worry about putting lots of hydrocarbons into the air (or their lungs). It says:
"If you'd like to treat a tent or tarpaulin with a good and proved homemade waterproofing solution, the job can be accomplished without too much difficulty. One of the best homemade solutions is simple to make and easy to apply. Just drop some blocks of canning paraffin into a gallon or two of white gasoline and let them stand for a few days or until as much paraffin as possible has been dissolved. The blocks may be eaten away entirely, or there may be nothing left but some thin, pock-marked paraffin wafers. About a pound of paraffin in a gallon of white gasoline is the right ratio, and you can shave the paraffin with a knife to speed up dissolving.
"Take this saturated solution out doors and -- keeping all cigarettes, sparks, or open flames well away -- paint it onto the tent or tarp. Use a clean, rather stiff brush, and work the liquid well into the material. particularly on roof, floor, and seams. The fluid will soak into the material and the gasoline will evaporate, leaving a coating of paraffin throughout the fibers. This treatment does not make the fabric too stiff, although it may stiffen slightly and shrink a bit as it dries. You can even use this solution on your convertible's top, and it will certainly make it waterproof. The white paraffin doesn't rub off much, and the color of the tent is not greatly changed, perhaps becoming a little darker.
"If you prefer to use some solvent other than gasoline, you can substitute either turpentine or benzene. But remember that these, too, are very flammable, so use extreme care.
"Probably the simplest of all home waterproofing methods is to rub block paraffin onto the material, section by section, as it's spread out on a smooth, hard surface. Once this is done, use a warm, not hot, iron to set the paraffin into the weave."
To this, I might add that you probably shouldn't use your mother's best ironing board.
I think this treatment was used on the tents we used in my Scouting days. As I recall, it had two drawbacks: it almost completely destroyed the breatheability of the canvas (not a problem for a shelter half, really), and when the material got really warm, like in the trunk of a car, the paraffin would tend to migrate, resulting in layers stuck together and generally uneven coverage.
For more information, first-hand experience, hearsay, and much confusion, you can read what others have posted on the Rialto on the subject. Consult Tanya Guptill's Medieval Pavilion Resources or Steve's Florilegium