I think stuff you are talking about is very similar or even same as RainX.
If you compare bottle sizes, 15ml to 500ml, £15 for 15ml sounds a lot, but the problem with spray on repellents is that they don't last long at all.
Plus you will have a lot of glass polish left (big bottle).
With this stuff process is much longer but there is a reason for it, it should last 20,000miles at least not only few washes.
The best way to do it is to get a kit.
Most of my time I spent on the big bottle, glass polish.
The coating itself needs just simple application of 3 coats, 15min wait after last coat, and then you just need to use residue remover (2nd small bottle) and you are done.
Value for money??
I think YES.
500ml will cost you same money, but this stuff needs to be applied on regular basis and thats where I would stop when choosing a good repellent.
If for the same money, but extra 1.5 hour of your time you won't have to do it for the next 20,000 miles, would you still use the 500ml bottle??
Some people would and there is NOTHING wrong with it.
It's all about personal preferences, and this stuff is really good....so far.
If I notice visible drop in it's function I will update this thread as soon as
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2006 D3 S8 5.2 V10
Ceramic Brakes, Bang&Olufsen, DTV, DAB, Rear Camera, BT, TPMS, Soft Close, Auto Boot, AMI , 4 Zone AC, Home Link, All Electric Blinds, Fingerprint, Ski Hatch, Black Extended Leather, Heated Sport Seats, Alacantra, Carbon Trim inc Aluminium Pack 2.
2015 A3 S line 1.4 TFSI - for the missus to keep her away from my car.
Standard S-line equipment, no extras.
2006 D3 A8 3.7 V8 - SOLD but not forgotten