Leaking tap and cant fix it, need help.

ive got a cold water tap in the bathroom, normal twist on off tap that wont stop leaking, it just goes drip, drip, drip, drip.
ive replaced the washer first time and didnt stop it, i got one of the seat grinding tools and gound it until it was flat and shiny and replaced ther washer and still didnt fix it, i put one of them domes washers in it and it fixed it for a month and now its dripping again.
what else can i do??
what i have noticed though is that there is like a hairline crack or mark of somesor on the seat i cant grind out with the grinder, could it be a crack and the water is leaking thorugh that??


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