Rust stains on block paving can occur from water running over rusty metal fixtures/fittings
Rust stains on block paving can occur from water running over rusty metal fixtures/fittings or from accidently spilling weed/moss killer containing ferric sulphate and is probably the most difficult type of staining to remove from paving blocks.
Many of the dyes used in colouring paving blocks are based on iron oxides so many of the chemicals used in cleaning rust may affect the colour your paving also.
General purpose concrete cleaners may help to remove rust stains, depending on the type of paving that is installed the effects can vary. Sometimes a little vigorous scrubbing with a wire brush and detergent solution can remove the worst of the staining. I’m afraid with rust; it is a case of seeing which solution works best on your type of paving.
There are a number of possible solutions. It is always a good idea to try it out on a small, indiscreet area first.
Lemon Juice or Vinegar: This method may not totally remove the stain, but can fade it so it doesn’t stand out as much. Apply the juice of a freshly squeezed lemon or clear, white vinegar. Leave it on the stained area for 5 minutes. While still wet, scrub the area with a plastic bristled brush and rinse well with water. Repeat again if necessary. Lemon juice and vinegar can kill or damage foliage, so please use caution when using these products around plants and bushes etc
NOTE: Do not use chlorine bleach on any rust stain; it will only make it worse.
Hydrochloric Acid: If the staining is severe, consider using a hydrochloric acid solution. Any acid needs to used very carefully and ideally it is recommended that you hire a professional to undertake the acid wash if you are at all unsure. If you decide to undertake the task yourself ensure that you wear the appropriate protective clothing – gloves, boots, goggles and so on – and always take extreme care.
To remove the stain firstly wet the area with clean water and then treat the area with a 7-10% dilution of a hydrochloric acid solution. When diluting acids, always add acid to water and not water to acid. As the staining begins to dissolve some frothing may be apparent. At this point agitate the surface by brushing a stiff bristle brush over the area to completely remove all trace of the stain. Once this process is complete the whole surface should be rinsed thoroughly with clean water, taking care to dispose of the run-off safely as soft landscaping can be severely affected by chemical treatments. If necessary, repeat the process – it is better to treat the area several times with the correct dilution than to use concentrated acid, which may damage the surface of the paving.