Mxaexm; as usual you get many answers. Depending on you model's size, and if glow powered or other should be your answer.

Brodak (http://www.brodak.com/) sells the best Butyrate dope going. It is a derivative of Randolph's aircraft dope. Sig Dope is simply a repackage of Randolph. There in the Chicago area you have Super Flite which is OK but requires a lot of thinner and reducer.
IMO, I suggest Brodak. You could order a pint of thinner, pint of reducer, and a quart of cope, or smaller dope for colors if you need them.
I have used a lot of nitrate, both by itself and and under buty. Don't do it. It will rot, especially silk, within 2 years. I use only buty now. Sig sells and I think Brodak does also, a non-tautening dope. However I rather use regular with a few drops castor oil per 4oz. dope. After you are satisfied with your paint job, use two light thin coats of the "plastisized" clear dope. That will stop any shrinking. Regardless of contrary information, buty will SHRINK big time, it just takes a month to cure.

If your FF scale model has a light frame work, be sure the covering grain is spanwise on the flying surfaces, and you may want to use just enough dope to tighten the material. One coat a day is plenty. Then wait several weeks befor applying glow fuel.

BTW, buty is glow fuel proof. Nitrate becomes like sticky goo when exposed to glow fuel or ehaust residue. [:'(]

Best of luck to you.