It is hard to let go of a method (and a skill) that you have been using for years. But I'm coming to realize that foam, at least how I'm using it, is a poor fit for the props I am doing today.
I met foam back in high school theater. It is still a great match for theatrical props, and that's what made it so hard to take a step back and see it in proper context. You see, theater props are meant to be seen from a distance. Forty feet, we usually say. And they don't have to last long or stand up perfectly to rough handling; many is the prop on many a show that finishes more tape and hot glue and hastily spray-painted touch-ups than original surface.
At the heart of most of the prop-making processes is a central paradox; you want a final surface that is hard and durable, but you want a material which is soft and malleable during the process of shaping that surface. Hardwood, metal; they both look great, but need power tools and excessive time to carve. Foam allows you to rough in a shape very, very quickly -- sometimes with nothing more than bare hands.
This flexibility can be a huge drawback, though. Foam is not quite durable enough for the kind of close-up hand props I'm making now. And it can't stand up to the rigors of vacuum forming, either. Yes, you can take a mold off it, and cast either a durable resin copy or make a better vacuum form buck from that. But even then foam is problematic for achieving a good final surface.
It beads, it cracks, it squishes. All those qualities that make it excellent for quick shaping, work against getting the final smooth surface and details you are after. There is a partial way around this; hold the foam back significantly from the final outlines, and coat it with Bondo. Then you can shape that more durable material with more confidence that it won't collapse.
Except you are basically trading away the advantage of foam being there at all. Bondo is further along the graph from "easy to carve" to "solid in the hand," enough that you are back into shaping with power tools. The foam is present in so rough a shape, it might as well be swapped out for cheaper and more durable MDF -- there's no loss in carving time since all the important shaping is taking place on the Bondo surface now.
I had a similar issue with FIMO, but the generally workable solution was to bake progressively; retaining the shapes that were finished so they didn't squash, distort, or get accidentally dented or otherwise mangled while you were sculpting the next element. (As an aside, Sculpey has this problem even worse; it is self-skinning, which is probably great for beads but makes it difficult to properly refine a surface without baking it then resorting to sand paper).
The ultimate along this line, to my mind, is that gift from the prop-making gods; Apoxie Sculpt. Apoxie fits well with my impatient approach and generally iterative workflow; it starts off soft enough to roughly prod into shape, like sticky clay. Then it sets up over about forty-five minutes in a balsa-like state that is generally too stiff to accidentally squish out of shape, but soft enough you can quickly whittle more refined details into. Then, in eight to twelve hours more, it sets to a durable solid like tight-grained wood, that can be drilled and sanded.
There are downsides to the stuff, but, really, the trade-off you get for materials like this, that are both easily malleable during primary shaping and tough and stable during the final detailing, is the cost of the stuff.
None of these "translation" materials are cheap, from Apoxie Sculpt to the silicone rubber used to make a decent mold. Which is why theater often doesn't translate -- it uses the primary carving materials (foam, balsa, foam-core, cardboard) as the final form. And, yes, people get excellent results with hand props and costume pieces with these materials, and similar. But it just isn't the kind of surface and durability I want for my own.
Well, there's a couple places where I can push closer to center. More expensive foams that carve tighter and better support surface finishing, for one. But I may have to give up on expanded styrene foam for all but massive, rough-shaped props.