Hi Digger,
I would first contact the selected Chromer for suggestions as his process may undermine the approaches suggested with the application of acids and heat. If you want to follow one of the suggestions provided (here), I would consider their application after the re-chroming but I would select an approach with very low or no heat to eliminate the risk of heat discoloration.
I do agree with the Healey-Nut that fitting the spears on installed fender is a PITA and I also talk from experience. However, I feel the spears should be applied on painted fenders and many have had their fenders installed on the car for painting. If the fenders are already mounted and painted, I would consider, proceeding with care, but dismount them for spear installation.
Just, some thoughts,
Ray(64BJ8P1)