Removing Lead Runoff from Glass: Chemistry, Coatings, and Costly Mistakes

Mark Cave

A customer rings you about a conservatory roof. Streaky white and grey marks running down the glass, right below a lead flashing. Your first instinct might be to reach for the softwash mix. Please don't. Because that stain is not green, it is not organic, and bleach is going to do absolutely nothing for it.

Mark Cave

I'm Mark Cave from SoftWash UK, and this one follows on from an earlier episode where we looked at metallic runoff under lead flashings on masonry. Today it moves onto glass. And glass is a much less forgiving place to get it wrong.

Mark Cave

Let's start with what you're actually looking at. Fresh, unpatinated lead is exposed to rain. The rainwater leaches it, and what runs down is a chalky mineral deposit, basically lead carbonate and lead oxide. That is an inorganic, insoluble metallic deposit. It's not algae. It's not biofilm. Sodium hypochlorite works by oxidising organic matter, so there's simply nothing for it to attack here.

Mark Cave

And it can make things worse. Sodium hypochlorite reacting with lead can turn the flashing brown or orange. So you've failed to clean the glass, and you've discoloured the lead above it. That's a fun phone call to make to the customer. Our own guidance is clear that hypo shouldn't be relied on for oil, rust, tannin, lead staining or heavy mineral deposits. Different problems, different chemistry.

Mark Cave

So what does remove metallic oxides? Traditionally, reducing or acidic chemistry, such as oxalic acid, or specialist acidic formulations. There are also fine abrasive polishing compounds, cerium oxide based ones like Unger Rub Out. On masonry, that's a fairly familiar route with a test patch. On glass, it's a different story.

Mark Cave

Here's why. Plenty of modern rooflights, conservatory roofs and Velux style units carry factory coatings. Some are self-cleaning coatings, the photocatalytic, hydrophilic kind, titanium dioxide based, Pilkington Activ being the well known example. Others are low-E thermal coatings. These layers are incredibly thin. Strong acid or an aggressive abrasive can etch, scratch, strip or cloud them. And once that happens, you can't clean it away. The glazing unit has to be replaced.

Mark Cave

Think about the numbers on that job. You were quoting for a cleaning price, and you've just created a glazing replacement. That's the consequence of treating glass like render.

Mark Cave

There's a second layer to it. Over many years, acidic mineral runoff can leach and bond into the glass itself. So you may be dealing with mineral etching rather than a surface deposit. If that's the case, no chemical on earth will lift it, because it's no longer sitting on the glass. It's part of the surface.

Mark Cave

So how do you approach it professionally? First, identify. Is it a surface deposit or a permanent etch? Is the glass coated? If you can't confirm what the glass is, treat it as coated and be cautious. Check manufacturer guidance where you can.

Mark Cave

Second, if you do consider oxalic acid or a specialist lead remover, start very weak. Small test patch, inconspicuous corner, strict control of dwell time. Watch it the whole time. The acid must never dry on the glass. And keep it off the rubber gaskets, aluminium frames and uPVC beads, rinsing immediately and thoroughly. Our Lead X Pro is designed for brown, orange and oxidised staining on lead surfaces, so it's really there for the flashing. Always read the label and the safety data sheet, and test before you go near anything else.

Mark Cave

Third, and this is the one people skip, deal with the source. If raw lead is still sitting above that glass, it will bleed fresh deposits on the very next shower. You can clean it beautifully on Friday and the customer's ringing you on Tuesday. Treating the lead, for example with a patination oil, helps seal the metal and slows the runoff. Otherwise you're just cleaning on repeat.

Mark Cave

Fourth, be honest with the customer. Results on glass can't be guaranteed. Document the existing condition with photos, explain the limitations before you start, and get a signed acknowledgement. Never promise like new glass. I'd rather lose a job than win one on a promise I can't keep.

Mark Cave

This is really what professional softwashing is about. Knowledge comes before products. A strong chemical and a good pump are useless if you haven't diagnosed the stain and understood the surface. Podcasts like this are a great start, but if you're serious, the CPD Certified Softwash Training Course at softwashing.uk goes much deeper into chemical selection, surface identification, safety and pricing work correctly. You learn online at your own pace, and you can become an Approved Softwash Applicator, with a directory listing and a place in the National Association of Softwash community.

Mark Cave

So here's your challenge. Next time you see white streaks under lead, stop before you reach for anything. Ask three questions: what is it, what is the glass, and where is it coming from? Answer those, and you'll avoid the costly mistakes. Take care, and I'll speak to you again soon.