Today I applied Optiglanz simply with a sponge, I applied it on each header pipe one by one and immediately rinsed it with a second sponge and plenty of water. It worked very well, the pipes are shiny now again.
Of course, due to the sloppy way I applied it the "giraffe stains" did not vanish. I will drive and wait some hundred kilometers and check whether the discoloration comes back again. If so, I will again apply it and try harder.
Attached two mobile phone made pics (sorry for the bad quality).