It depends on what they are supposed to represent.
I am not happy with my wooden buildings, the outbuilding is a bit featureless, and the dacha look a bit plasticky.


Slaters corrugated embossed plastic roof (possibly 4mm scale).
I actually covered a 4Ground shed with Slater's embossed plastikard, but have not actually photographed it.
More Slaters embossed plastic and tar paper made from tissue paper.

For materials such as brick, concrete, plaster etc, Plastikote Rust Effect spray (or similar - there is a "suede" spray available from one manufacturer which is recommended by 7mm to the foot modellers for concrete surfaces) is your friend.
If it is a concrete or fully rendered building, use filler (I use Milliput) to cover joins etc.

This is one of Sally 4th's pill boxes. It is the same construction (stacked slices) as this one.

But the surface has been smoothed with Milliput.
For the Vietnam buildings, I smoothed the joints with Milliput, masked and sprayed the building with Plastikote Rust Effect spray, then Halfords Grey spray Primer and Citadel Corax White.

Another building masked:

And "rusted".

For a Rendered/adobe brick jail (40mm). Use the filler to generate a thickness of render over the bricks.

One of the key things is to add details to buildings. If you are looking at modern buildings aircons are great.

Carlos has mentioned there should be a fan size hole in the wall.
A detailed description here (even if it is Plastikote and not rustoleum):
https://ultravanillasmurf.blogspot.com/2018/10/sarissa-small-terraced-house.html