If you have already tried the power washer, this is normally your first stop, If this hasn't worked, have you tried it again? Sometimes takes a few goes to really make a difference
Failing this you could also try bleach. Brush some bleach on, leave it to sit for 10 mins or so and then give it a good scrubbing (wear protective clothing, rubber gloves and eye protection when doing any of this though). Once scrubbed, wash off with clean water. This normally gets off a fair bit but in some cases not all of it.
If you have any remains left then you should then be able to sand this off but you may need to leave it to dry out thoroughly first though
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