Can I use this to repair a cinder block that surface is flacking

Thank you for posting your inquiry on DAP Ready-To-Use Concrete Patch. DAP Ready-To-Use Concrete Patch can be used on all common masonry surfaces, such as concrete, CMU ("cinder block"), brick, and similar materials. Please note that the "flacking" (spalling) you describe is often a signal of a larger underlying condition, which may require different repair materials/methods. Without more background/details, we cannot provide a more definitive response/recommendation in this format. Should you wish to provide these details, so that we can provide you with more specific product usage guidance or to recommend a more appropriate product, please feel free to contact DAP’s Technical Customer Service Department at 888-327-8477.