Preheat and clean the grates

Brush them hot

The manual's every-cook steps.

1. Preheat: all burners on high, lid closed, 15 minutes.
2. Open the lid.
3. Brush the grates straight after preheating, in heat gloves.
4. Check the grates for loose bristles before food goes on. A bristle-free brush (our pick, the BBQ Daddy) has none to lose.
5. Set the knobs for the cook.

The manual: buy a new stainless-bristle brush at the start of every spring, and replace it at once if it sheds. Change a bristle-free brush's head when it wears.

Deep clean them cold

Weber UK's gas barbecue cleaning page.

1. Let the grill go cold and turn the gas off.
2. Lift the grates out.
3. Brush both sides, spray them with grate cleaner and wait 30 seconds.
4. Scrub with the rough side of a sponge, rinse and dry.
5. Lay the grates back in.
6. Preheat on high 10 to 15 minutes before you cook.

Some darkening and staining is normal (Weber UK).