Good morning Tony
We used Mequairs Gold Class clay kit on our XK X150 which seemed pretty perfect. Nevertheless, a film was lifted leaving a perfect base for application of Carnauba wax afterwards. If the car has had Superglaze or similar applied in the past, then it may be harder work to get rid of that.
The Mequairs kit comes with the spray on lubricant...essential to use this or the clay will drag (and also be really hard work!). In fact, the kit contains everything you need.
Also essential to apply a non-abrasive wax afterwards....again, we used Mequairs Carnauba which resulted in a really smooth finish.
We took about 3 hours to do our XK with the clay kit, another hour and a half or so to finish with the Carnauba. As stated in earlier posts, there is lots of advice on the net.
Well worth the effort and you can then join the Sparkly Club!
Happy growling