How do I stop channeling in espresso?
Channeling means water found a low-resistance path through the puck instead of extracting it evenly. Fix it with prep: stir the grounds with a WDT tool to break clumps, tamp level, and make sure your dose actually fits the basket.
You can see channeling before you taste it. Pull with a bottomless portafilter and watch: a healthy shot starts as separate streams that converge into one steady stripe within 8 to 10 seconds. Channeling looks like spray from one point, a jet firing sideways off the basket, or a bright blonde patch appearing early while the rest of the basket is still dark. In the cup it reads as sour and harsh simultaneously, because part of the puck is under-extracted and part is over-extracted.
The cause is density variation in the bed, and almost all of it is created at dosing time. Every grinder throws clumps at espresso fineness, and clumps sit next to loose pockets. Water goes where the resistance is lowest. The single highest-return fix is WDT: sink thin needles, 0.3 to 0.4 mm, into the dosed grounds and stir in small circles from the edge inward, reaching nearly the bottom of the basket, for about five to ten seconds. Do not overdo it, since aggressive stirring drags fines to the base of the puck and creates its own dense layer.
Second, get the tamp level. Absolute pressure matters far less than people think, anything from roughly 10 to 20 kg gives similar results, but tilt matters enormously. A puck tamped a degree off level has a thin edge that water will exploit every single time. Tamp on a flat surface, look at the puck from eye level, and if you cannot keep it flat by feel, a self-levelling or palm tamper removes the variable entirely.
Third, check headspace. There should be a small gap between the tamped puck and the shower screen, roughly 3 to 5 mm. Too little and the screen imprints the puck and disturbs the surface; too much and the puck can lift and crack when water hits it. If your used puck comes out with a deep circular imprint of the screen, you are overdosed for that basket. If it comes out soupy with a crater in the middle, you are underdosed. Match your dose to your basket rating, usually 18 g in an 18 g basket, and only then start moving it.
If prep is clean and channeling persists, look at hardware. Cheap stock baskets have uneven hole patterns and thick, tapered walls; a precision basket is a genuinely worthwhile upgrade at this point. A puck screen on top of the bed can also help distribute the incoming water on machines with aggressive, unsoftened flow. And check the basket for dents, since a bent rim or deformed base creates a permanent channel.