This may be a new idea but it’s more likely an old idea that I’ve rediscovered. Since it’s new to me, it might be new to enough others to make it worth posting.
Never mind how I discovered it but I learned recenty that a mixture of salt, baking soda, and water is very difficult to remove from a smooth surface after it dries. This worthless bit of information came to mind when I needed to weather a covered hopper that had been hauling cement.
Voila!! A small patch of salt and baking soda beside one of the hatches looks exactly as though some workman spilled cement while loading the car and then rain turned it into concrete. I cheated a bit by using a thinned water soluble paint (Poly Scale’s Concrete) instead of water to wet my cement mixture but it worked perfectly. That covered hopper now has a permanently attached patch of spilled cement/concrete.
I think the ratio of baking soda to salt is about 4:1 but that’s a wild guess. I’m not even sure the salt is necessary but that’s what my wife said was in the mess I was trying to clean up.
Chuck