You can seal cinder blocks with a sealant, which is a type of paint that helps protect the blocks from water damage. Sealants are available at most hardware stores. First, you have to clean the blocks with a power washer or hose. Then, you apply the sealant with a brush or roller. Finally, you let the sealant dry for 24 hours before using the blocks. Sealing cinder blocks is an important step in protecting them from water damage. Sealants help to keep moisture out of the pores of the concrete, which can cause the blocks to crack and break over time. Applying a sealant is easy and only takes a few minutes, but it's an important part of maintaining your cinder blocks.