Gotta go with the "industrial" stuff.

Also, there's stuff called tanglefoot that's used to control insects on trees or other gardening situations. Expensive in bulk though.

The key with both of those options is delivery and target.

They'd do fine on masks and firearms, but deploying them is a pain in the rear, particularly when churn syrup is cooked down into something seriously difficult to deal with. You can water balloon regular corn syrup in a pinch, and be fairly accurate with practice.

However, when you have something that sticky, area matters. Using the cooked down syrup or the tanglefoot over a larger area to making crossing it on foot difficult is likely a better option since it can be done at a greater distance from any moving person, or ahead of time to provide movement control on flanks where barricades wouldn't be viable or last as long.

But, yeah, straight corn syrup, if placed well, is going to slow anyone down, and not just their walking speed. You'll have trouble seeing if it hits a mask or face it reduces speed of reloads, and even of other operations of firearms/grenade launchers. It can make deploying flash-bangs harder. But you are stuck with someone nursing needing to get close enough, or being good enough with a longer range deployment method to get it on target. Potato guns can make distance well, but aren't very accurate or fast to reload.

I bring that up because most average people aren't going to have access to anything else that can lob a breakable container any real distance without also breaking the container before it can reach the target. So, you'll be delivery by hand with a very fragile container most of the time, or using improvised launchers.

Improvised launchers vary in range and accuracy, but aren't super accurate at all. Elastic bands to make a big slingshot tend to work okay with liquids in rubber; they don't usually break the thicker balloons that can be used for this kind of substance, but they're really skill dependant if you want to target a person. They work fine against groups though.

Combustive launchers like potato guns tend to break flexible containers, and that immediately takes the launcher out with things like this, so it would need to be reserved for non flexible containers, and they would need to still be light, so you'd be using smaller loads. But, it can go farther, so it's not useless.

Beyond that, you're getting into complicated builds that aren't going to be carried by one person, so it's not applicable for this kind of deterrent

Now that's new to me! Nice stuff, it makes total sense. It's cheap, fast, easy to throw.

Gotta go with something environmentally safe, but not biodegradable.

Mica flakes would do the trick. Reflective enough, dyable in a pinch, easy to source.

Drawback is sharp edges, but that can be mitigated with a decent suspension in slime. The stuff that kids were all into a while back. Non toxic, easy to make in big batches, sticks all over, and hard to clean off while still on the move.

But, the slime itself is a usable option tbh, the mica glitter would just make cleanup harder, it wouldn't add much to is effectiveness, or I don't think it would.

I've seen regular glitter vs masks, and it does work by itself, to a limited degree. So the mica may well be a good option as an addition, I just don't know when I'd be able to test it, so I can't say for certain. I can ask around, see if I can find someone to work on it as a fun experiment.