You need to enable custom depth stencil in your project settings. After that, you can apply almost all of the effects to actors with the specific stencil ID(s). All effects have an “advanced” section (dropdown) in the details window. You will find the masking options there but first, -as I said earlier- you will need to go into the editor project settings under rendering ->post processing and set the “custom depth stencil pass” setting to Enabled with Stencil.