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Replies

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  Geoff Norfolk May 8, 2010 at 8:24am

  Not oil spots..... some sort of algae I suspect........ and they seem to like red paving slabs and not the grey (patio was a mixture of the 2!) I've also had this on block paving where you get a few dark black 10p size dots that won't shift.
  
  Steve Kenyon The Garden Company said:

  are they oil spots ?
• 

  Geoff Norfolk May 8, 2010 at 11:07am

  I've 3 gallons of that kicking about..... I'll try it on the tough bits and see if anything happens..... don't hold out to much hope! Incidentally, if I hold the nozzle a couple of cms away from the spot and blast it for 30seconds or so, it does go but I'll be there forever!!
  
  colin said:

  mmc pro
• PRO

  Geoffrey King May 8, 2010 at 1:41pm

  You could always try dabbing 'brick mortar cleaner' on the black spots
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  Kieran Ray May 8, 2010 at 4:55pm

  That does move them, but can damage the slabs. The spots are a litchen.
  
  what is the psi of you washer?
  
  Geoffrey King said:

  You could always try dabbing 'brick mortar cleaner' on the black spots
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  Geoff Norfolk May 8, 2010 at 6:59pm

  I'll give that a go......... its only a few small patches. My washer is only about half the pressure I'm afraid...... 130psi....... but I have the dirtblaster lance/nozzle which boosts things up a bit. Would love to have a megapowerful one like yours but my wife would kill me!!
  
  Pro Gard said:

  Sodium hyperclorite (available from farmers merchants) wet the slabs, spray on neat via knapsack, leave 1/2 hour then wash. Wear a respirater, face shield and waterproof trousers when you spray it as it is a clorine based bi product of bleach manufacture and will burn your lungs if you breath it in.
  
  A better (industrial) pressure washer will however lift them without chemicals, mine has no problem ( 250 bar, 15 lPM 3600 psi with turbo lance and 13 hp honda engine)
  
  No chemicals used on either job:
  
  [IMG]http://i240.photobucket.com/albums/ff163/paul1f/f291.jpg[/IMG]
  
  [IMG]http://i240.photobucket.com/albums/ff163/paul1f/f288.jpg[/IMG]
  
  [IMG]http://i240.photobucket.com/albums/ff163/paul1f/df002.jpg[/IMG]
  
  [IMG]http://i240.photobucket.com/albums/ff163/paul1f/ptcl001.jpg[/IMG]
• PRO

  Will - WB Design & Construction May 8, 2010 at 11:03pm

  Intresting to hear how you use hypochlorite pro, i teend to dilute it in a watering can and poor it on no pre wet then.
  
  Pro Gard said:

  Sodium hyperclorite (available from farmers merchants) wet the slabs, spray on neat via knapsack, leave 1/2 hour then wash. Wear a respirater, face shield and waterproof trousers when you spray it as it is a clorine based bi product of bleach manufacture and will burn your lungs if you breath it in.
  
  A better (industrial) pressure washer will however lift them without chemicals, mine has no problem ( 250 bar, 15 lPM 3600 psi with turbo lance and 13 hp honda engine)
  
  No chemicals used on either job:
  
  [IMG]http://i240.photobucket.com/albums/ff163/paul1f/f291.jpg[/IMG]
  
  [IMG]http://i240.photobucket.com/albums/ff163/paul1f/f288.jpg[/IMG]
  
  [IMG]http://i240.photobucket.com/albums/ff163/paul1f/df002.jpg[/IMG]
  
  [IMG]http://i240.photobucket.com/albums/ff163/paul1f/ptcl001.jpg[/IMG]
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  Geoff Norfolk May 15, 2010 at 9:13am

  Had a quick search...... swimming pool maintenance suppliers sell it for about £20 for 25 litres.
  
  Steve Kenyon The Garden Company said:

  can you still get hypo ?
  
  ROWLY HILL said:

  Sodium hypochlorite - I used to use gallons of this stuff in milking parlours. Question though as I've got a patio to clean next week. Any likely damage to grass as the slabs butt up to the lawn or is it diluted sufficiently when pressure washing to be of no harm...I want a clean patio but I defo don't want a scorched lawn edge.
• PRO

  Will - WB Design & Construction May 16, 2010 at 5:06pm

  Try mole valley if you have one in your neck of the woods or a simmilar farm supplier should be much cheaper than that.
• PRO

  Will - WB Design & Construction May 16, 2010 at 5:27pm

  I paid something like £7 from MV the other day Country wide is normally that bit more expensive.
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  Dave Sewell May 17, 2010 at 8:48am

  Back to the cause of the problem- it sounds like black lichen and we find it's especially prevalent on Indian stone. In fact we've found it to be such a problem that we now try to avoid using Indian stone altogether if we think the client won't regularly pressure-wash the paving. It's a devil to shift so this thread is making very interesting reading!
  Dave