Atsko, amongst others, make a spray for that.
http://www.atsko.com/products/waterproofing/silicone-water-guard.html
I used the McNett version in this clip..
http://www.youtube.com/user/francodarioli?feature=mhee#p/u/12/GBnkGIz9cco

They used to have a liquid version (1 gallon can) but cannot see that on their web site now.
I think that the liquid one is 100% silicone (like the GEII) highly diluted with mineral spirits.
I use something like a 1 to 2-3 dilution for seam sealing (1 part of silicone) so I would use something like 1 to 5 for reproofing.
If the stuff sack of your shelter is made from silnylon do a patch test on that.
BTW, there are less than 2 oz of silicone in those cans, so you do not need all that much to do the all tent.
I would just hose down the tent first. If really dirty than wash down with pure soap and water (the stuff you use for wool/down)
Franco