BSJ to ASJ

I made a Baby Surprise Jacket as a stripe prototype for an Adult Surprise Jacket which I am making for myself. I don't want any light colored stripes going across my bum! I am using the Lionbrand fishermens wool in the natural color and in the natures brown. I want my fabric stiff to wear it as a jacket, not a cozy sweater. So far 21sts /4" (5.25sts per inch )on size 7 needles is too loose. I am going to make a swatch using size 6 needles. I won't go lower than that cause then I"ll never finish the jacket. When I figure out how to post pics I will.