Stain the surfaces of concrete or cement walls in an unheated garage with acid stain for a look that allows the surface material to show through. Clean the walls with a TSP cleaner and rinse well with a garden hose before staining, and then apply the stain from the top to the bottom with rags. Use a push broom to etch the stain into the wall, and wipe any stain that remains on the surface off of the walls after the amount of time that the stain manufacturer recommends on the stain packaging.
Paint both ceilings and walls in an unheated concrete or cement garage with masonry paint if you want to cover the wall material completely. Clean the walls with TSP cleaner first, and then apply masonry primer over the surface. Paint the ceiling and wall surfaces with two or three coats of paint until the concrete or cement no longer shows through.
Lower the ceiling of an unheated garage, and get a finished look, by installing a drop ceiling. Install the wall angles and brackets for drop ceilings using masonry or drywall screws, depending upon the wall material, and anchors, and follow the manufacturer’s instructions to install the grid work for the ceiling and place the tiles.
Texture the walls and ceiling of the unheated garage if you would like to change the flat surfaces. Mix a batch of thinset mortar or mud, and prime the surfaces with paint primer to give the mortar or mud something to which to cling. Fill a texture gun with the mortar or mud, and spray the material onto the walls to create a uniform pattern. Apply the mortar or mud with a putty knife randomly to create a less uniform appearance.