As the previous poster suggests multi-layer platinum/palladium printing was used extensively by Irving Penn, the article referred to in the original post I wrote in the spring of this year, for those people interested you can download a copy
here
As I suggested in my concluding remarks
A multi-layered approach to platinum printing not only increases the density of blacks possible but gives the platinum printer greater control over the final print. This is a results of being able to craft individual negatives for each successive layer, using differing exposures times
as well as using different contrast mixes of platinum and palladium for each layer.
Multiple hit printing is not needed for every type of image and as mentioned in the article '
exquisite results can be achieved with a single hit approach,' however for certain types of images you can achieve results that are simply not possible with a one hit as Penn found out early on in his Platinum printing experiments, this again i refer to in the article and is shown below :
His initial results using a single layered approach to platinum printing were in his own words as being less than satisfactory. Through extensive research and testing he was able to replicate the process, but not in spirit. However in an interview for the book between Greenough and Penn in 2003 he told her of the moment he had an epiphany, she states
Persistent, Meticulous, but also intuitive, Penn had a breakthrough while he was working on a television commercial for Nescafe. As he looked at the first platinum print he had ever made he realised in a flash that he needed to coat, expose and develop his print multiple times in order to achieve the richness and complexity he desired. Also he immediately understood that in order to coat and develop his print several times, he needed to ensure that the paper did not change size during its repeated submersions in chemicals. To do this, he realized, the paper had to be attached to a stable material, such as aluminium.