I've found that fondant is easiest for me. Dries hard and will keep it's colors if kept out of sun/light and cover from dust. I've tried covering a 14" with royal and couldn't get it smooth. Wilton instructor said to do the royal very thin and pour it over the dummie. Would probably need several coats. I'm trying that next. Check my photos - the whimsey cake and baby shower cake are both cake dummies, decorated totally with fondant.
I've done both fondant and buttercream - both are still holding up fine! For an easy buttercream (psuedo buttercream), this might work better than your royal... use crisco, water and powdered sugar. (The recipe I have is for 5#s: 2# crisco, 3/4 cup +? water, 5# sugar). You can obviously modify). It goes on really smooth and dries nice and hard.
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