If you change to concrete and sand, or mortar mix with an acrylic additive available in the concrete isle, you have a substrate intended to be outside. Dry wall mud with latex paint simply will never have enough acrylic medium unless you change it to 4 gallons of paint to one gallon of drywall mix. Which would be a mess.
In the stucco family there are base coats that already have the polymer in the mix in a dry form hence the greater expense as the goodness is in there. Adding your own liquid additive and tints are better for workability and storage of materials where moisture might ruin your supply of stuff.
When you get right down to it, it might be twice as expensive as Monster mud but, haunters generally only need a few square feet worth of stuff. You are protecting your labor investment by using better materials.
It can soak burlap if that's what you want or fiberglass cloth or just be smeared over a wire mesh and built up, carved etc but it is something intended to be outdoors with or with out sealants.
Another fabulous post from the U.S.Department of Wild Imaginings, now in spectaclar stereo, sponsored by the Adhesives and Sealants Council, suggesting ways to stick things together since the 1800s. Not fabulous in a gay way. Your results may vary. Illinois residents add 8% sales tax. These posts have been made by professional post makers, do not try this type of posting on your own without extensive training, lovely assistants and a trusty clown horn.