A few years ago I had my back yard paved over but every winter it gets covered in green algae. At the moment its so thick it looks like a lawn !!!! I have a pressure washer and try and keep it clean but with this weather its getting worse. Don't get much sun out there so that doesn't help I think. Is there a varnish or something similar I could paint over them. I have painted the walls white but its starting to appear on them as well. Any advice would be appreciated.

You could try washing soda or Jeyes fluid.There are many substances on the market which you can pour onto the paving to remove and prevent growth.B&Q or any DIY place will have various concoctions.

As Salmon says, there are several products on the market, but ordinary bleach is cheap and will usually do the job.

This'll sound strange, but fill a cheap spray bottle with white vinegar and spray it on the algae. After you've sprayed it on, just leave it. It won't look as though it's done anything but after a few days the algae will start to lift off and clear itself. No scrubbing required.

Vinegar ? Amazing uses for it.

http://www.vinegartips.com/Scripts/pageViewSec.asp?id=6

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Where can you buy full-strength white distilled vinegar ?
I'm guessing its not for your fish n chips?

Request Acetic Acid - chemists???

As with Chris.... bog standard cheapo bleach from Tesco's, Asda, Lidls et al. Used it for years on north facing paved areas etc. Pour it on and brush gently to cover all the area. Leave for a few minutes. Use a hard yard brush and scrub (no need to go mad). Swill off with plenty of cold water. Warning... worms in the immediate run-off area DON'T like it ! It also retards the return of any green gungy stuff* for quite a while.

* Apologies for the use of detailed scientific description!

I just used the cheapest stuff i could find in the supermarket. So long as it's acidic - which is what (I presume) kills the green stuff.

I bought white vinegar for household use last year. As I was at Light Cinema at the time I bought it in New Brighton but cannot remember if in Home and bargain or Morrisons. Not used it since but suppose handy to have in.