I used silicone caulking to treat heavy paper mailing tubes to seal and make waterproof. Worked great.
Mixed 3 to 1 low odor mineral spirits to silicone caulking by weight. Wasn't real accurate, didn't need to be. Mixed it up in small batches using only 1 oz of silicone caulking. Found out that a little bit goes a long ways.
I've read where commercial 1.1 oz silnyl is actually 1.3 oz per yard. So figure 0.2 oz per yard silicone. That's 5 square yards per oz of silicone. The JRB 8x8 tarp is slightly over 6 square yards, so 1 oz of silicone will just about do the whole tarp. Figure 1.5 oz would be more than enough.
As warbonnetguy says it takes a long time to mix it thoroughly. Once mixed I simply used those foam brushes to paint the inside and outside of the tube. Hung in the basement to dry. Took about 24 to 48 hours to cure really good. Tried a second coat on one, but the second coat ended up peeling off in some places. The first coat penetrated far enough in the heavy and dense paper and prevented the second coat from penetrating real good.
Did the same on some heavy cordura fabric. Same mixture. 2 coats worked real well. Again just mixed and painted using foam brushes and allowed 48 hours to cure thoroughly.
Water just beads up and rolls right off.
With the low odor mineral spirits, neither my girl friend (whose nose is very senistive) nor I could smell anything in the house at all. Also, don't worry about time. It took a long time, many hours, for it to dry and cure. Even the excess left in the tuna fish mixing can.