Water Butt Problem

[Paul]

During the summer (think there was a couple of weeks of sun somewhere during that time), the water butts got a healthy dose of Algae.
So I've been scrubbing them clean and jet washing them ready for the winter when, as always, our mains water trickles to a stop.

Comes with living on the end of a VERY long water pipe I suppose.

Problem is the 120 liter butts still feel 'slimy'.
Clean but slimy.
First time it's happened in 5 years.

Anyone got a quickie cure for this?
Please don't say flush them with hot water though, with 120 liter barrels it's just not practical.