It is said that a semi-static rope should be submerged for 24 hours before first use.
Why is this, and is this before every first use ?
Yes, it is accepted practice to wash new Semi-Static rope (or Single Rope Technique/SRT Rope as it is known by cavers).
Dave Elliot is a highly respected SRT expert, and he wrote the CNCC Rope Care page, which says:
There are two reasons why new ropes are best washed before use. Washing removes the anti-static lubricants used in manufacture and also shrinks the rope. This serves to compact the sheath and tighten it onto the core, stabilising the rope and perhaps improving its wearing properties a little.
When first soaked all ropes, particularly nylon ropes, can be expected to shrink by various amounts up to 8 or 10%. Obviously it is as well to roughly determine the shrinkage before cutting and marking ropes for length. Never the less, the rope will continue to shrink (at a much lower rate) throughout it's life, mainly due to the effects of the mud which inevitably penetrates the sheath.
They shrink, apparently. For example, Beal's Precautions says:
Before first use, soak the rope and leave to dry slowly. It will shrink by about 5%. Take this into account when calculating required lengths.
And first use means just that: The very first time the rope is used.