The first CIPP liner installed in London in 1971 was pulled into position within the host pipe and then inflated and cured. However, within a few years, an inversion installation process had been developed in which the impregnated liner was contained within a plastic handling layer and was unfurled (inverted) within the host pipe using water or air pressure. This method causes the liner to unwrap itself within the host pipe without the need to drag the prepared liner through the host pipe. The handling layer remains on the inside of the finished liner and the impregnated felt/fabric winds up in direct contact with the host pipe. In some cases (notably when heavy groundwater inflows are present), a pre-liner can be inverted within the host pipe and then the actual liner inverted within the protection of the pre-liner – preventing washout of the resin before curing. In the pull-in process, there is a handling/sealing layer on both the outer and inner surfaces of the liner. This allows the liner to be pulled in place and then for the inflation and curing process to be accomplished from inside the liner. Both methods are effective for thermal curing and the choice can depend on job circumstances or contractor preference. Ultra-violet curing always follows the pull-in process.