Most of these things people read about or experimented with and pretty soon people thought they had to add to their mixes. You really don't need them for small projects.

Arcylic admix is just another name for arcylic fortifier and you find it where you buy your bags of portland cement. Comes in liquid form and one manufacturer is Quikrete.
Fiber mesh is available from masonary supply stores.
I think there's a project in Sherri Hunter's book where she uses steel wool. Can't imagine why anyone would put steel wool in their projects. Have you ever seen bridges that are falling apart and inside the cement the rusted steel. The rusted steel is what made the cement fall apart.
You asked in another thread what was leaching. It's removing the lime from your cement project. I use a water bath. It's a plastic muck bucket filled with water. After I unmold and clean my project, I place my project in the water bath. I change the water every day for 2 or 3 days by then the lime is leached out. You're removing the lime because some people and plants are sensitive to lime. The people can get a nasty skin burn if they touch a wet planter that hasn't had the lime removed. Some plants can die from too much lime. Billie

Thanks so much for answering my questions, including the one about leaching. What I wasn't quite sure of about the leaching really had to do with how soon after a piece is taken from a mold it is placed in the water. Having never done hypertufa, I just wasn't sure how hard it was at that stage. It sounds like it's plenty hard to put in water right away. I have one more question.
What is the purpose of acrylic fortifier? Added binding/strength?

I saw an article about shredding fine steel wool fibers into concrete (mainly on the exposed surfaces) to create a look similar to marble. I think the thin fibers rust and then fall away or disintegrate leaving the look of veining. Don't know if that would work as well with hypertufa. I've also seen concrete floor installers/stainers randomly sprinkle ironite onto the surface of a floor that's just been finished but still wet in order to get a mottled stone look. After it has rusted (usually left overnight) they wash away any remaining ironite and then stain over that.

I unmold in 24 hours for many things, clean them up or carve then place them in the water bath right away. You have to be very careful because their very fragile in the first 24 to 48 hours.
Here's a link to Quikrete's web site with info about their fortifier and other products they manufacture.
Sorry, when you said "shredded up steel wool as a mesh" I thought you meant in the mix not on the mix. Fiber mesh is used in the mix not on the mix. There are tons of things that you can use on the mix as a decorative element.

Just curious, what are you making? Billie

Here is a link that might be useful: Quikrete

Haven't yet decided what my first project will be. I've thought about creating hypertufa 'blocks' (about 20 inches high and wide) that have a horizontal groove in them near the top of the block to receive wooden bench slats/boards.
In other words, the hypertufa blocks would be at the two ends holding the bench 'seat' boards. That's why I've been curious about it's strength and longevity, and whether it
was common practice to have a metal frame of some sort internally (like for concrete) for structural strength.
It just seemed like garden furniture would require more strength than a pot or other common uses of hypertufa.

If you're making bench legs you can use concrete & rebar for strength not fibers or acrylics.