How Long Should D/2 Dwell Before I Start Scrubbing?

headstone cleaning in Florida | headstone cleaning in Pennsylvania

I’ll never forget my first time using D/2 Biological Solution, it was a hot July afternoon down at Oak Hill Cemetary, and I was staring at this beautifull old marble headstone for a woman named Clara Bell—died 1887, lichen crusted so thick you couldn’t read her name. I sprayed the D/2 on, waited maybe three minutes because I was impatient, and started scrubbing with a soft brush. Took me an hour. My back hurt. The stone came out okay, but not great. I left thinking, "There’s gotta be a better way."

Turns out, there is. The secret is patience—specifically, knowing how long to let D/2 dwell before you even touch the stone. I learned that lesson the hard way, but you don’t have to. After six years of volunteering and cleaning hundreds of stones, I’ve got the timing dialed in. Let me walk you through it, so you don’t waste an afternoon scrubbing like I did.

The Short Answer: 15–30 Minutes, But It Depends

Most manufacturers say 15 to 30 minutes for D/2 to dwell before scrubbing. That’s the sweet spot. But here’s the thing—that’s for a light cleaning. If you’ve got thick black biological growth, like moss or lichen that’s been there for decades, you might need an hour or even two. I’ve let it sit overnight on a really stubborn sandstone slab and it worked beautifully. The key is to not let it dry out completely. If it’s a hot day, I’ll mist the stone with water every 15 minutes to keep the D/2 wet. Dry D/2 is useless D/2. Big mistake I made early on—let it dry on a sunny granite marker, and I had to reapply. Waste of time.

Why Dwell Time Matters More Than Scrubbing Pressure

D/2 is a biological cleaner, not an abrasive. It works by breaking down the organic matter—algae, fungus, lichen—at a chemical level. Scrubbing is just helping lift the dead stuff off. If you scrub too soon, you’re basically pushing live lichen around with water. It’s like trying to mop a floor before the cleaner has had a chance to dissolve the grime. You’ll just smear it. I’ve seen people scrub so hard they scratch the stone, especially on softer marble. Let the D/2 do the heavy lifting. Your arms will thank you. For most stones, I spray it on, walk away and clean another marker nearby, then come back after 20 minutes. That’s my rhythm.

How to Tell When It’s Ready to Scrub

You’ll see the growth start to change color. Green moss turns brownish. Black lichen gets a sort of grayish, flaky look. That’s the biological matter dying and loosening. Another sign—if you touch the surface gently with a gloved finger, the growth should feel soft, not crusty. If it’s still hard, give it more time. I had a limestone headstone once that took three applications over two hours because the lichen was so thick. Patience paid off. Also, if you’re cleaning a stone with carved lettering, pay extra attention—growth in the grooves takes longer to penetrate. I’ll sometimes use a soft plastic scraper (never metal!) to gently lift loosened bits after dwell time, but only after the D/2 has sat for at least 30 minutes.

Common Mistakes and How to Avoid Them

First mistake: using too little D/2. You need to saturate the growth, not just mist it. I use a garden sprayer set to a fine stream, and I make sure the stone is completely wet with the solution. Second mistake: scrubbing with a stiff brush. Stick with a soft nylon brush or even a natural bristle brush. Wire brushes? Never. They gouge stone. I’ve seen it ruin a 150-year-old marble marker. Third mistake: scrubbing in direct sunlight on a hot day. The D/2 dries too fast. Early morning or late afternoon is best, or on an overcast day. Fourth mistake: using D/2 on a stone that’s been sealed or treated before. It won’t work as well. If the stone has any kind of coating, you’ll need to remove that first. I usually test a small area first.

My Go-To Process for Most Stones

Here’s what I do now, every time. First, I brush off loose dirt with a soft brush. Then I wet the stone with plain water—just to cool it down if it’s warm. Then I spray on D/2 from bottom to top (so I don’t get drips on dry spots). I set a timer for 20 minutes. If the stone is in shade, I check it at 20 and usually start scrubbing. If it’s in sun, I check sooner and might mist with water to keep it wet. I scrub gently in small circles, focusing on the carved letters. Rinse with a low-pressure spray from a spray bottle or a hose without a nozzle—just let water run over it. Never use a pressure washer. I’ve seen too many stones pitted from that. Rinse until no bubbles or residue. That’s it. Simple. Effective. Took me years to nail down, but now I can clean a typical headstone in under an hour, start to finish.

Frequently Asked Questions

Q: Can I leave D/2 on a headstone overnight if the growth is really bad?

A: Yes, you can, but only if the stone is in full shade and the temperature stays cool. Overnight works great for thick lichen on granite or sandstone. But on marble or limestone, I’d be cautious—prolonged wetness can sometimes cause spalling if the stone is already fragile. I’ve done it maybe a dozen times, always with good results, but test a small hidden area first.

Q: What if the D/2 dries out before 15 minutes? Should I reapply?

A: Absolutely reapply. Dried D/2 becomes inactive. Just spray a little water on the stone first to rewet it, then apply more D/2. On hot days, I’ll mist the stone with water every 10–15 minutes to keep the D/2 wet. I learned that one the hard way when I let a batch dry on a granite slab and ended up with white residue.

Q: Do I need to scrub at all, or can I just rinse after dwell time?

A: For really light growth—like a thin layer of lichen or algae—you can sometimes just rinse after 30 minutes. I’ve done that on newer granite markers. But for most old stones, a gentle scrub with a soft brush helps lift the dead organic material out of the pores and carvings. Skip scrubbing and you might leave a film that looks dull.

Q: How do I know if the stone is too fragile for D/2?

A: If the stone is flaking, crumbling, or has deep cracks, skip the D/2 entirely. D/2 is a water-based solution, and any water can worsen existing damage in fragile stones. For those, I just dry-brush with a soft brush and leave them be. If you’re unsure, check with a cemetery preservation pro or your local historical society.

Q: Can I use D/2 in cold wheather?

A: It works best between 50°F and 90°F (10°C to 32°C). Below that, the biological activity slows way down, and dwell time might need to be doubled. I tried it once at 40°F and it took nearly two hours to see any change. Not worth it. Wait for a warmer day.