An excessive amount in my opinion, saturate the applicator and wipe on thick making sure everything gets coated in the first layer, wipe on in a criss cross patter and wipe off after 20-30 seconds.
I've also done the plastic trims in front of the window and all the plastics on the clio, just like in
@Hixle picture. That worked perfectly and they are looking as new, so I don't think it's product or user error.
And as
@JamesBryan says, Gyeon Trim and Gtechniq C4 are pretty similar stuff.
I'll see what the cerakote wipes are like, but either way, it's a £1000-1500 on a good day so not the end of the world.