Cinder blocks are tougher than bricks, and can usually withstand the etching force of pressurized water or sand, but neither power-washing nor sandblasting is a safe method for removing lead-based paint from a wall that predates 1978. Even if the paint isn't lead-based, using a chemical stripper is often safer, faster and less messy -- especially if you use it in conjunction with a peel-away membrane. If you're only trying to remove graffiti or overspray, you may be able to do the job with trisodium phosphate.
Stripping with Chemicals
The procedure for stripping cinder block and masonry walls is virtually the same as that for stripping paint from any material at the beginning of the job, but because cinder block is so porous, the end of the job -- the part where you remove the paint residue -- can be difficult. You'll probably ultimately have to use a power washer, but since most of the paint is gone, you won't have to expend as much effort as you would if you relied on it completely. The use of a peel-away membrane helps the stripper stay moist longer, which allows it to dissolve many coats of paint at once.
Paint Stripping Procedure
Cover the floor under the wall with a disposable drop cloth. If there are any plants or landscaping features in harm's way, cover them with plastic. Put on goggles, rubber gloves and protective clothing.
Spread a thick coat of masonry paint stripper on the wall with a stiff paintbrush, being sure to work the stripper into mortar joints and other crevices. If you have a peel-away membrane, apply it over the stripper as soon as you're done spreading it.
Wait for the paint to start lifting; depending on the stripper you use, this could take anywhere from one to six hours or more. If you didn't use a membrane, you'll see the paint begin to bubble and crack. If you used a membrane, look underneath it periodically -- the paint is ready to strip when it has liquified and comes off with the paper.
Peel the membrane -- if you used one -- and scrape off the paint, using a paint scraper. Scrub mortar joints and stubborn paint patches with a wire brush. If you applied a membrane, much of the paint will come off when you remove it, but you'll still have to scrape and scrub. Collect the paint residue and put it in contractor bags for disposal.
Apply a second coat of stripper if there is still a significant amount of paint on the wall. Let it work for an hour or two, then scrape it off.
Remove residue by power washing. Keep the pressure of the power washer between 1,500 and 2,400 pounds per square inch, and use a 25- to 40-degree tip. Maintain a nozzle distance of at least 6 inches to prevent the high-pressure water from gouging the cinder blocks.
Neutralize the stripper by washing the wall with a product recommended by the manufacturer. In many cases, clear water does the job, but some caustic strippers may need to be neutralized with an acid.
Removing Graffiti and Overspray
You usually don't have to resort to stripper when removing spray paint from cinder blocks; instead, you can usually wash it off with trisodium phosphate, a strong detergent which acts as a mild paint stripper.
Mix a solution consisting of 1 cup of TSP per gallon of water. You'll need to wear rubber gloves and goggles while working with TSP -- it can cause caustic burns.
Scrub the painted area with a stiff fiber-bristle scrub brush soaked in the solution. It may help to scrub an area, leave it for about 10 minutes, then come back and scrub again.
Rinse with clear water after you've removed all the paint.