Don't forget that the FK 425 is really just a QD product to maintain your work.
For FK, I'd go the 2180 topped with Pink Wax.
I had a bright silver car for about 4 years and I tried the Klasse twins (w/multiple layers of SG) also topped with P21S and I also tried AIO with multiple layers of PB's EX-P and also tried the P21S over that and ultimately, the products that made me the most happy were Zaino.
I even tried to top the Zaino with P21S and Souveran but it was the pure Zaino that did it for me on silver. Multiple layers of Z2 just made it pop to my eye.
I just got rid of that car but I would love to have tried the Werketatt products,Prime followed by Acrylic Jet. I am awaiting those to try on my Titanium Grey car which does look nice now with AIO,PB's EX-Px2 and P21S but I want a bit of a change.
Let us know what you choose!
Eric