Creating dirt on buildings for city scene problem / Texturemask propably wont do it

Hey Community,

i wanted to create a texture mask in Blender for my buildings (dirt dripping down the windows and on the surface) to blend two materials in UE4 together. My mask right now is 4k big, but the mask is stil pixelated. I think, going to a higher resolution wont cut it and with 8 individual buildings, i will propably get memory issues.

I need high res textures for my dirt. I found a tutorial for decals, but placing a decal for every window will take a huge amount of time.

Is there anyway i dont know about?
How would you do it?

Please send help!