We get a lot of questions about bottomless portafilters and getting sprays (or little jet streams) coming from different directions while pulling a shot. This is a common, so if you experience it, don't worry, a lot of people do (even experienced baristas from time to time). We call this channeling. Channeling is frustrating and affects flavor by resulting in a very sour and/or bitter shot (not to mention the mess it leaves).
If you are nodding your head in agreement because you know exactly what we are talking about, here are a few reasons of why:
1) Your bed of freshly ground coffee has been tamped down at an angle, leaving a surface that is not level. Think about gravity, and then add force (9 bars of pressure)...the water naturally will go through the path of least resistance. Thus, water will want to shoot through the shallow side, while on the deeper end it will be coming through in slow drips.
2) Another problematic area to really be careful of is around the rim of the portafilter basket. If you haven't spread the coffee all the way around making sure there are no gaps around the edge of the portafilter, the water will find a path at the rim of the basket and go through quickly.
3) Air pockets, divots, and cracks in the coffee bed can also be the culprit. For example, if your ground coffee has some clumps in it, this can create air pockets, and thus all kinds of weird channeling.
There is a solution to each of these issues.
First, make sure you settle and distribute the coffee grounds. Settling refers to using force by tapping the portafilter vertically (or horizontally) to collapse the fluffy mound of coffee. This gets rid of the air pockets, and helps evenly distribute the coffee grounds. Once you tap the portafilter a few times, you'll want to focus on making the coffee bed level and flush with the basket’s rim. Hold the portafilter firmly on a flat surface and use a straightened forefinger to push the mounded coffee toward one edge of the basket (think of leveling off a cup of flour, except that you are not pushing the mound off the edge). Allow some part of the rise to remain, a decreased hill, but still a hill, near the edge of the basket. Repeat this motion in four directions – away from you, toward you, to the left, and to the right. After each pass, the mound is smaller. After the final pass, any remaining coffee can be pushed off the edge. Be sure that where coffee and basket meet, you have left no gaps. [Note – if you are single dosing, you will not have excess coffee grounds, but you may still level the coffee grounds using the same technique and make sure that the coffee grounds come all the way to edge, all the way around].
Next we tamp. You will want to create the most uniform bed of coffee so that it can evenly resist the significant force of pressurized brewing water for an even extraction. Hold your portafilter firm and level on the tamp mat or stand (it’s important that the portafilter basket is flat, and that you are not tamping at an angle). Grasp your tamper as if you are shaking hands with a doorknob, and apply light, even, and equal force to the coffee bed. Use the sides of your thumb and forefinger to gauge if the tamper is level with the basket’s rim. You need not be overly forceful – comfortable and firm is best.
To recap, 1) prep your coffee bed to get rid of air pockets, gaps, and cracks by settling, 2) distribute everywhere so the water won't try to sneak through around the rim of the bed, and lastly 3) give a solid even tamp to make sure you create even resistance for the perfect shot.