Hydraulic cement is generally used for patching water-related cracks and holes in concrete, but it also makes a good patching alternative for any type of cement repair. Although the chemical properties are slightly different than genuine cement, it can be painted over just like regular cement. However, there are a few preliminary steps to take before a finishing coat of paint can be applied.
Mix up some trisodium phosphate or other heavy-duty cleaner in a bucket with water per the package instructions.
Scrub the hydraulic cement with a brush that is to be painted. This will remove all oil and dirt and make the cement a good surface to hold primer and paint. Allow to fully dry.
Duct tape a piece of plastic over the dry hydraulic cement and allow it to stand overnight. Remove the plastic the next day and look for any condensation that may have built up. If there is any moisture present, the cement will need to be sealed before painting by brushing on a surface water sealant and then allowing to dry thoroughly.
Brush or roll on a latex or oil-based paint primer. Smaller areas can be done with a brush, but a paint tray filled with primer and then rolled on will save time over larger areas. Allow the primer to dry.
Cover the primed area with a finish coat of paint, your choice, by either brushing or rolling it on. Thinner and multiple coats give better results than one or two thick coats.