Hi Shaun,
I made a tray to lay the pieces in and done a test on the two small pieces that sit on the front door pillars to make sure I got it
strong enough, left it for about 30min then took it out and rubbed it down with a scouring pad and left it for another 30 min.
Bearing in mind i had already wet and dried most of it first you might want to leave it longer & make it stronger.
you will know when its off coz it returns to its normal untreated state (White with lots of light scratches.) You will know.
But its knocked 3/4 off the time id guess!!
Give it ago and let me know how you get on Shaun.