Why Scotchgard? Is that a real waterproofing agent? I have used Nikwax spray waterproofing before, to reasonable success. In any case, I use a heavy duty plastic clear bag as a liner and my down sleeping bag gets it's own "plastic bag-inside-the-stuff sack" routine. Never had wet or damp down in my life -- "knock-on-wood".

There is mostly a threat of dampness spreading from the moisture inside the clear bag so I separate any damp clothing from the rest with another stuff sack.