Use deferred decals. You can add anything from leaks, dirt, snow, exposed bricks, even objects like this trap…
Any material works so you could make pretty cool stuff. Here I just used a simple diffuse decal texture from cgtextures.com
I don’t know if they are performance hungry so use them wisely.
Small vent exit
Hey. What are you using to light the scene? When I tried to use it in my scenes earlier I came up with a problem. It was bearly visible while using static skylight. It only worked for me with dynamic direct light. I found this solution Decals not rendered without direct lighting? - Rendering - Unreal Engine Forums but it still didn’t work out for me.
Dose anyone have the idea how to solve this problem??
My lighting is only a static skylight with HDRI. For the decal material I used the emissive slot instead of base color. With the multiply node I can gauge the intensity.
That’s really cool! Do you have to uv those into the mesh or can you just place them wherever? I’m not familiar with decals though the name implies just stick them wherever.
The decals itself doesn’t need any uv mapping. Now if the mesh you apply it on need one, i’m not sure. All my meshes were uv mapped anyway for my materials. I don’t think its a necessity tho.
I experimented with POM decals too, but the limitation that no one seems to be able to get around is that they can only be placed on the ground. If you rotate them on any axis, the effect breaks. Still would work really well for potholes and sewer gratings and whatnot, however.
Really nice feature. I’ve used them in my archviz projects to simulate some dirty on the walls too 
