Plaster vs. Spackle vs. Molding Paste
The comparison was interesting.
Patching Plaster: I liked the way the plaster went on the best of these three. It felt creamy, like a slightly runny frosting. It took awhile to dry; you can check the status by touching the surface--if it's cool to the touch, it's still drying. The most interesting thing about the plaster was when I painted over it--it dissolved/disappeared, revealing part of the painting underneath that I was covering up! I loved the serendipity of it--what was revealed was an eye--ooooh! Other than that, it was completely opaque. This was the easiest of the three to carve back into after it was dry.
Spackle: Much stiffer to spread than the plaster, the spackle nevertheless spread well. It starts to dry fairly quickly so there's not much time for reworking the surface. Due to the stiffer texture, the spackle works exceptionally well with stencils. Spackle holds paint well after drying and is less absorbent than the molding paste or plaster.
Golden's Light Molding Paste: This stuff is great. It spreads nicely with a softness somewhere between the plaster and the spackle. It works well with stencils, is not water soluble after drying, and is the only one of these that is translucent after drying. It also actually absorbs paint that is later applied on top of it, but with an interesting twist: the paint can be removed with a damp cloth--absorbed right back out of it. The light molding paste dries as quickly or more quickly than the spackle, but does not 'carve' as well as the plaster or even the spackle, due to its flexibility or rubberiness. this flexibility makes light molding paste the best choice for surfaces that are very bendable, such as large stretched canvases or large pieces of paper.