Spray, Coat, or Dip? Maybe try this?
WalMart sells Atsko "Silicone Water-Guard" in a spray can in the Camping section. It will spray/impregnate a LOT of fabric for roughly $5.
This has been working for me ...
Buy WalMart's $1.50 yd polyester or nylon coat lining material which is roughly an ounce a yard. Create/sew your project, and spray Atsko's Sil Water Guard on the fabric and you just might be impressed how it repels driving rain on a tarp. Not as good as factory SilNylon or the Spinnaker fabrics, but again, you just might be impressed with performance/value.
I've even found DWR at Wally @ $1.50 a yd. Lot's of coated camo 2 oz yd lately. Better hurry tho, chek EVERY time you are in Wally because, the clerks tell me all fabrics will be gone from Wally in a year or less. Then it'll be www retailers @ $5 -$10 or more a yd.
FYI: I've been making what I call ''hybrid'' tarps (ie" water PROOF 54" tarp top coverage with 18" bottom sides sewn to lengthen the tarp's overhang coverage.
You can save a LOT of money this way if you are buying expensive spinnaker fabrics (ie: Cuben or Spinntex. Plus, it adds maybe an ounce or two with the finished overall weight. This will save you 3 Meters or so of that "high dollar" fabric assuming your tarp is roughly 10' long.
Isn't the HF wonderful?
Value gear eingineer