I'm interested in your experience with top dressings.
I've read about how moss can be difficult in our climate and solutions for making it work in the Parameters for growing moss chat. I don't think I'm ready to mess with that, yet.
Still, I am having issues with soil washing away when I water āĀ particularly on store-bought, deciduous trees that have more of a traditional soil, instead of the akadama mix I've used on others. So, I'm looking for ways to cut that out.
I have one tree from The Bonsai Nursery with a kind of sharp, small, black rock on top. I can't tell if it's lava or not, but it seems to work really well without the bulk of chunkier lava I've seen around.

Perhaps the solution is going back and asking what they use, but I'd still like to know more.
What do you use?
Here a couple of photos of using Sphagnum moss held down with galvanized metal wire brackets and a New Mexico Privet under going my Moss Growing Technique which was started around 2.5 weeks ago. There are some lichens growing in the moss, which was intentionally done by me.
To close the loop here I went by Bonsai Nursery yesterday to ask what they use. The material is called charcoal breeze rock. I was able to get a sandbag full at Ewing Landscape Materials for less than $2.
https://www.ewinghardscape.com/product/breeze-rock-charcoal/
I think the key to growing moss on the front range is to find moss at our elevation. I tried collecting mountain moss, and it hasn't taken, but I found a patch of moss at park near my house (literally 100 yards away) which has taken hold and started to grow using the Mirai method.
Come grow moss in my garden Mike!
Sphagnum moss cut up and held in place by galvanized brackets pushed into soil. Nothing hard about growing moss in Colorado. https://rockymtnbonsai.com/index.php/bonsai-basics/moss-information/moss-growing-in-full-sunlight/
This tree was the most touched tree in the show last year. People just had to touch the moss.
You can buy window screen by the roll from Walmart/Ace/HD/Lowes. I'm still using a roll I bought a few years ago.
With your Dramm, you can turn the water flow down on your input from the house, you can turn the spout upside down while you're holding it, and you can practice watering in waves instead of keeping it in one place the whole time.
Kind of like you don't leave a hair dryer in 1 spot for long or you get burned.
All that said, soil will still wash out sometimes.
If you watch through this watering video from Mirai you can see what I'm talking about in several of the clips. They also grow moss pretty well. :)
Other things that will help: 1. A really fine watering watering spout.
2. Control while you're watering to avoid flooding.
3. Leaving a "ditch" around the the outside of your pot, slightly below the rim of the pot. This allows a little movement, without the soil washing out.
4. If you fertilize with anything that contains or is humic acid, the soil particles eventually stick together pretty well.(on my trees) This shouldn't be confused with soil going hydrophobic!
5. You can use window screen on the top of the soil, and this can also help prevent washouts.
-Spaghnum technique doesn't work in my yard very often.-