I've created a decal of a toxic waste spill, and I was wondering if there's a way I can lay a static mesh on top of the decal without the mesh being affected by it?
Yeah, that will disable decals for that one mesh. That'll work.
If your needs are more complex in the future though, definitely look at the Decal's properties. Look under the "Decal Filter" section. In there, if you set "Filter Mode" to "FM_Affect" you can then fill out the "Filter" array with just the meshes that you want it to be affecting. You can also do things like toggle off BSP and terrain...
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In the static mesh's properties > StaticMeshActor > Rendering > [check off] Accept Static Decals.
If your needs are more complex in the future though, definitely look at the Decal's properties. Look under the "Decal Filter" section. In there, if you set "Filter Mode" to "FM_Affect" you can then fill out the "Filter" array with just the meshes that you want it to be affecting. You can also do things like toggle off BSP and terrain...
Good luck!