spraying from a bottomless portafilter usually means water is finding an easy path through the coffee instead of moving through the whole puck evenly. that is called channeling. it is a puck prep issue, not a sign that your machine or portafilter is broken.
a bottomless portafilter makes channeling visible. with a spouted portafilter the same thing can happen, you just never see it. so treat the spray as useful information about the shot, not as a new problem.
what is channeling?
water under pressure follows the path of least resistance. if one part of the puck is looser, lower, or cracked, more water goes through that part. the rest of the puck gets less.
that uneven flow hurts the taste. the easy path over-extracts and the dense parts under-extract, so one shot can taste sour and bitter at the same time. the mess on your counter is the smaller problem.
once a channel opens during a shot, it tends to get worse as the shot runs. fixing it happens before you lock in, not during the pull.
how do i fix the spray before brewing?
start with distribution. clumps and uneven piles in the basket are the most common cause. break up clumps with a distribution tool or a thin stirrer, or tap the side of the portafilter gently to settle the grounds. the surface should look level before you tamp.
tamp level. a puck that is lower on one side gives water a faster lane on that side. check the surface after tamping and redo it if it is tilted.
be gentle when you lock the portafilter in. banging it against the machine or wrenching it hard can crack the puck, and a crack is an open channel.
what if it still sprays?
look at your dose and grind next. a dose that is too low for the basket, or a grind that is too coarse, leaves the puck without enough resistance. a slightly higher dose or a finer grind often settles the flow.
a grinder that produces inconsistent grounds at fine settings can also cause it. if every other variable looks right and the spray keeps happening, the grinder is worth checking.
if your machine has a pre-infusion setting, use it. wetting the puck gently at low pressure before full pressure gives the coffee time to swell and seal small gaps.
change one thing at a time and watch the bottom of the portafilter. even, centered streams that join into one flow are the goal.