Can I pressure wash my dirty block paving before sanding and sealing?

The main problem with pressure washing them now is that the water is going to go straight through those open joints and saturate your sharp sand bedding layer. If that sand bed gets soaked, you're looking at several days of perfect weather before you can even think about brushing in your kiln-dried sand. If you try to sand it while the blocks or the joints are even slightly damp, the sand will just clump up on the surface and won't drop down to fill the gaps properly. You’ve also got to worry about the pressure shifting the sharp sand underneath. Without the jointing sand in place, the blocks aren't locked togather yet, and a high-pressure stream can easily wash out the bedding or cause the blocks to tilt. It's a bit of a catch-22 becuase you obviously don't want to seal in the dirt, but forcing water into the sub-base this late in the game ussually causes more headaches than it solves. A better approach would be using a stiff broom and some **soapy water** or a mild acid-based brick cleaner to shift the worst of the muck. Scrub them manually and then just use a garden hose on a light spray setting to rinse the suds away. It won't get deep into the substrate like a power washer would. You need those blocks to be bone dry—not just on the top, but all the way through—before you even open the bags of kiln-dried sand. If you're dead set on using the pressure washer, keep the wand at a shallow angle and never point it directly down into the gaps. But honestly, if you seal them while there's still moisture trapped in the sand or the blocks, you'll end up with **efflorescence**—those white, cloudy stains—trapped under the sealant. That's a massive pain to fix later, so give it way more time to dry than you think it needs.

That efflorescence stuff sounds like a total nightmare to fix. I’m about to start a similar project and didn't even think about the moisture getting trapped under the sealer like that. If I go the soapy water route instead of the pressure washer, does the type of soap matter? I’m worried about some wierd residue sticking to the blocks and making the sealer peel off later.

Also, if it happens to rain right after I scrub them, am I back to square one waiting a full week for it to dry out? Trying to time this with the weather seems impossible.

Stick to a basic pH-neutral detergent or a tiny bit of washing up liquid. Avoid anything with built-in waxes or 'shine' additives, as those leave a film that prevents the sealer from bonding properly. If it rains, you're stuck waiting. To be 100% sure it's dry enugh for the kiln-dried sand, tape a piece of clear plastic over a few blocks for an hour. If you see fogging or droplets under the plastic, it's still holding moisture. I've seen plenty of people ruin a batch of sand by being impatient. Better to wait an extra day than end up with a sticky mess in the joints.

I used a stiff wire brush on some old reclaimed blocks last summer. It's a massive workout for your back but way safer for the sand bed than a power washer. If you've got stubborn cement bits or old moss, a hand scraper works wonders. Just be careful not to gouge the surface if the blocks are a bit soft. After you finish the wet scrub and it finally dries, give it a real thorough sweep with a soft bristle brush. You want every bit of loose dust gone before that kiln sand goes in, otherwise the dust mixes with the sand and makes a right mess.