The reason you should paint the cinder blocks is that it will help preserve the wood posts. By painting the blocks, you are waterproofing them.
If they aren’t painted, they are absorbing water, so the wood will be sitting in holes that are constantly wet when it rains.
Even though the posts will be stained and coated with urethane, they will be much more likely to rot. Painted cinder blocks will dry much more quickly and wick away much of the water.
How to Make a Cinder Block Bench Step by Step Tutorial
Step 1. Wash The Cinder Blocks
Place the cinder blocks on top of something so that they aren’t sitting on the ground. That way they don’t end up sitting in their own dirt and can dry more quickly.
I had square cinder blocks leftover from another project that I used to raise them.
Add TSP to the soap canister in the power washer, or fill a 2 gallon bucket with water and ¼ cup of TSP and wash down the blocks, if hand washing, use the scrub brush to help remove dirt and oil.
