Why do my windscreen wipers smear instead of clearing?

Smearing is nearly always a contamination problem rather than a worn-blade problem, which is why new wipers sometimes smear straight out of the packet.

A wiper works by dragging a thin, sharp rubber edge across the glass at a slight angle, flipping direction at each end of the sweep. Anything that stops that edge making even contact produces streaks. The three usual culprits are a dirty blade, a contaminated windscreen, and a blade that has hardened with age.

Start with the glass, not the blades. Windscreens accumulate a film of road grime, exhaust residue, tree sap and — most commonly — traffic film mixed with the oily residue left by old wiper rubber. Ordinary screenwash does not remove it. Cleaning the outside of the glass properly with a dedicated glass cleaner, or a mildly abrasive product such as a clay bar or a smear of car polish buffed off completely, transforms wiper performance more often than new blades do.

Then clean the blades themselves. Lift the arm, hold the rubber and wipe along its length with a cloth dampened with screenwash or isopropyl alcohol. The amount of black residue that comes off a blade that looks clean is usually a surprise.

If both are clean and it still smears, the rubber has hardened. Wiper rubber perishes from UV exposure and heat, typically lasting six to twelve months. A hardened blade will not flip cleanly at the end of each sweep, leaving a characteristic band of untouched water.

Juddering rather than smearing points at the arm angle or a seized pivot, not the blade.

A hydrophobic glass treatment helps in heavy rain but makes worn blades chatter more.