Coffee Appreciation

My shots keep coming out waaaay too fast, i've made the grind finer but still coming out way too fast, I can only assume it's a result of the tamper not covering all of the portafilter so when I tamp it doesn't compress all of the coffee so I need to tamp a few other bits too. Leaving some bits where the water can just rush through. I'm getting myself a better tamper anyway so hopefully that'll sort it but has anyone got any other ideas?

what you are describing is called 'channelling', but it doesn't usually effect a shot pull very drastically, in fact you'd probably only notice it with a bottomless (naked) portafilter. the 20 bar of pressure that gets run through your grouphead is equivalent to about 300 kilos on your tamper. what is your grinder?