A cinder block chimney is not exactly attractive. You can cover it up with an application of stucco, which will give it a more appealing finish. Stucco is similar to concrete. It has cement in is as well as sand, but it also has lime, which creates a slightly different texture. You can apply the stucco material to the cinder block surface in the same manner as plaster to create a more interesting look.
Pressure wash the concrete cinder block chimney to remove dust and debris. Let it dry for two or three days.
Screw 3/8-inch wood strips along the sides of the chimney and down the middle to help guide you when spreading the stucco so one side doesn't end up being thicker than the other. Use masonry screws to attach the wood.
Obtain a stucco mix at a home improvement store or a building supply store for the top finish layer and mix a batch for the base layer. You need cement, lime and fine sand for the base layer.
Mix 1 part cement, 1 part lime and 4 parts fine sand in a container to create a base layer. Add water to create the consistency of a thick pudding. Use a wheelbarrow to hold the mixture.
Spray the cinder blocks to dampen them.
Spread the base layer on the chimney to a thickness of the wood strips so that the stucco is even with the top of the strips.
Place a flat edge over the strips and drag it to remove excess stucco. Unscrew the wood strips and fill in the space with stucco. Looks aren't an issue since this is the base layer.
Wait for the stucco to lose its wet sheen and rub the surface with a foam float. Swirl the float over the stucco to prevent future cracking. Let the surface dry for one day.
Mix the package of finishing stucco and apply it to the surface at a thickness of about ¼ inch. Spray the surface to dampen it before applying the finish. Damp stucco bonds better than dry.
Spread the stucco on with a trowel in sweeping or swirl patterns to create a textured surface or spread it on smooth for a flat surface.