What to look for when choosing coffee for filter
Filter coffee is less forgiving than espresso in one specific way: there's nowhere to hide. No milk, no pressure, no crema to soften the edges. What you taste is the bean, the water, and the grind, and not much else. That's either a reason to be careful about what you buy, or a reason to stick to instant. We'd argue for the former.
Roast date, not best-before date
The most important thing on a bag of coffee is the roast date. Coffee is at its best for filter brewing between about ten days and six weeks after roasting. Before ten days, the CO2 hasn't fully off-gassed and the extraction will be uneven. After six weeks, the volatile aromatics that make a good filter coffee interesting have started to fade.
Most supermarket coffee doesn't carry a roast date, only a best-before date, which tells you very little. Specialty roasters always print the roast date. If a bag doesn't have one, that's useful information.
Roast level and what it means for filter
Light and medium roasts tend to work better for filter than dark roasts. Dark roasts have had more of their original character roasted out and replaced with roasty, bitter notes that can become harsh in a long extraction. Light roasts preserve more of the fruit and floral notes from the original bean, which come through clearly in a V60 or batch brew.
The house blend at Sparrow Windmill is a medium roast, which is a deliberate choice: it works well as both espresso and filter, which matters when you're running a small cafe with one grinder on the bar.
Grind size and brew time
For a V60, the grind should be medium-fine: finer than table salt, coarser than espresso. The brew time for a standard 15g dose should be somewhere between two and a half and three and a half minutes. If it's running faster, grind finer. If it's stalling, grind coarser.
At home, a hand grinder is a reasonable starting point. The Hario Slim or the Timemore C2 are both consistent enough for good results without spending a lot of money. The main thing is grinding fresh, immediately before brewing.
The current single-origin on the V60 at Sparrow Windmill is a washed Kenyan from Nyeri. It's worth trying black if you usually take milk.