Yes, I am pretty sure it is 1/2 ounce / Sq yard. Most foamie slope soarers I know of are covered with packing tape and filament tape underneath. The filament tape is required for rigidity in the wing and fuse. I am not sure you would get the same kind of strength from 1/2 ounce FG. I have used it on a blue foam project and it is very wispy stuff. But, blue foam is very rigid to begin with, and I think the FG was more for abrasion and preventing cracking the foam than for rigidity. I could be wrong, but I think a heavier FG would be required. Also, if you've never used FG before, it requires probably more resin than you think, thereby negating your weight savings. I suppose if you want to know for sure, find a very accurate scale, and mock up a square foot of material, and weigh it before and after the covering. The difference is the weight of your covering. It would be interesting to see the difference in weight for different resins as well. Be sure to include any coats of paint you will need as well.

Also, if you use epoxy for your resin, be sure to use the finishing resin as it is much thinner and easier to use. Sometimes it can even be too thick for very light FG. Use a squeegee to get the excess resin out of the cloth. Read a book called "Canoe Craft", it has a great treatise on applying glass and epoxy resin to canoes, and the info applies directly to the use of it over wings.

I'm not sure, but I think you can dye epoxy resin and possibly eliminate coats of paint on your model. West Systems would probably have this info.

Having said all that, most slopers have fairly high wing loadings and unless you just want to build it for the lightest wind conditions possible I think the covering would make a negligible difference. Use what is easiest, most cost efficient, and most importantly, most appropriate for the application. ie if your plane is EPP foam and you fiberglass it , the FG will crack when you crash it because the foam will compress then rebound yet the rigid FG won't. Whereas, packing tape and filament tape will give with the foam. In any case, good luck.