Rust stains arise from water running over metal fixtures and fittings then depositing iron particles on the paving and/or from the oxidation (rusting) of iron-rich compounds in the aggregate of the paving. There's also a danger of accidental staining when weed- and moss killers containing ferric sulphate are applied to lawns and gardens next to paths, patios and driveways.

Rust stains are probably the most difficult to remove from most types of paving. Many of the dyes used to color concrete paving are based on iron oxides, so many chemical agent may affect both the rust and the dyes. The 'patio cleaners' sold at the DIY sheds are based on a hydrochloric acid, and these usually have no effect on rust, although the stronger acid dilutions can sometimes make the stains darker.

Due to enormous differences in the type, nature and porosity of any substrate, there are a number of possible 'solutions' that may work. It should be noted that just because a particular 'solution' works well on, say flagstone, that does not mean that it will work on bluestone, or on decorative concrete. Similarly, any particular solution may have different effects on wet-cast and pressed concrete flags. Whatever is used, it's best to try it out in a small, discreet area first, as it's impossible to predict just how any of these will react with any given stone or concrete.

Lemon Juice - genuine lemon juice, fresh from a lemon, not from a bottle or a plastic imitation lemon. Squeeze directly onto the stained area, allow it to react for 5 minutes or so but don't allow it to dry out. While still wet, scrub the stained area with a nylon/polypropylene bristled brush (an old toothbrush is ideal), working the lemon juice into the surface of the paving. Wash off with plenty of clean water after 5 minutes and repeat as necessary.

Vinegar - use a clear or white vinegar (spirit vinegar) rather than a colored vinegar (malt or balsamic). Some readers of this site have reported that cider vinegar gives good results on some concrete surfaces. Use in exactly the same way as described above.

Oxalic Acid - nasty stuff, as it's highly toxic, and it's not easy to find. It's more commonly used in the woodworking trades to bleach or whiten wood, but it is quite effective at removing heavier rust stains from concrete and some types of stone. It may come as a proprietary "rust remover" in which case you should follow the manufacturer's instructions to the letter. The efficacy of these products is highly variable and they MUST be tested on an inconspicuous area prior to use to ensure that the paving is not damaged or discolored by their use.

General purpose concrete cleaners may also help to remove rust stains, but again, they are incredibly variable in effect, and it might be as well to use just a simple detergent and hot water.

All this leaves mechanical action as the only non-chemical remedy; vigorous scrubbing with a wire brush can sometimes remove the worst of the staining, as can 'buffing', a process where a hard stone or abrasive is used to 'sand down' the surface of the paving, removing the rust, and revealing a clean surface beneath. As ever, prevention is better than cure. Although rust staining does not pose a threat to the structural integrity of the paving in the way that oils do to tarmac, it can be unsightly and detract from the aesthetics. Removal of rust sources and/or use of drip trays will keep most of the stains off the pavement. If using a moss killer, then ensure any adjacent concrete surfaces are covered and protected.