TheBruce Mueller

I neglected when I commented the other day to address something in the original question: Can I use another oil - peanut, olive, coconut, tung?
Of those, the only one that polymerizes, making it a permanent, as opposed to temporary, waterproofing, is tung. I've not compared the cure
times, but what I got in a quick search seems to indicate that raw tung might well be slower than linseed. There's a polymerized variety which
cures quicker, but that likely would have the same downsides as "boiled" linseed oil.

Mineral oil (e.g. baby) comes from petroleum (the "mineral"). I wouldn't think, again because it doesn't polymerize, that it would be a good
long-term waterproofer.

I haven't yet used it, but a commercial product that seems very useful, and relatively cheap, is Nikwax Cotton Proof. I was told by them that
for my 20 oz/yd (yes, too heavy, lol) canvas, to use the same application rate as lighter material. So 1 L diluted and sprayed can easily treat
my 450 sf... 50 ml will treat 30 sf (3 m^2), so a liter is good for 600 sf. At $42/L, + $10 shipping (free shipping over $50), a bargain in my book.
A key to success, regardless of the treatment, is ensuring your canvas is clean. Mine is new; as a precaution of any processing residues, I plan
to wash with very dilute dish soap. With Nikwax, or other water-based treatment, you want the canvas damp, definitely not bone dry. On the
contrary, with the oil/solvent (e.g. classic wax/linseed oil/mineral spirits) type, the drier the better.

This is long after the previous posts. As a chemist, I thought I'd throw in my 2c worth.

First, what is now called "boiled" linseed oil is not boiled. Metallo-organic compounds (e.g. containing cobalt or zirconium; older was "Japan" dryer) are
added which greatly accelerate the oxidation/curing/polymerization of linseed oil. It is unsaturated, that is has double bonds; this is what allows it to cure.
Now, if you want a natural, food-grade alternative, you can actually boil RAW linseed oil. DO NOT DO THIS WITH "BOILED" LINSEED OIL. LIKELY YOU WILL
HAVE A FIRE!! Remember, rags that have been soaked with it will spontaneously combust; heating it will accelerate that.

Second, turpentine is distilled from pine sap. Now, as far as its safety is concerned, be concerned. It isn't distilled from petroleum, but it is still quite toxic if
ingested. Inhaling it short-term can cause e.g. headaches. Long-term, or repeated, can cause allergy/sensitization, so PLEASE act accordingly. Mineral spirits
have an advantage, less likely to cause sensitization. The flash point is slightly higher (5F): no open flames with either, unless you're using as lighter fluid to
start your bbq.