You might want to run "The Hone Test for plate sensitivity"
In order to perform properly, a printing plate's background must be hydrophilic; attractive to water and repellent to oil. If the background of the plate is not adequately hydrophilic, all one can hope for is that it will require more water be run in order to keep the background clean, interfering with the ink/water balance. The plate background does not have to be stained or discolored in order to be inadequately hydrophilic to print well, sensitive plates can look OK, but still print poorly. There are several tests for the performance of the background (placing drops of solvent on the plate, deletion pens, droplet tests, etc), but all have potential shortcomings except for the 'Hone Test'.
To perform the Hone Test you need a product referred to as 'scratch remover'. We will define a scratch remover as a product designed to repair scratch damage to the plate by re-silicating the background. In order to be suitable for the Hone Test, a scratch remover will need to contain silicate in some form and have a pH above 10. Check the label and/or MSDS, there are lots of products like this on the market (not all are labeled as scratch removers, many plate cleaners and other types of products meet these criteria) and as long as these two conditions are met, you are good to go. Do not let anyone try to convince you that gum, plate cleaner, plate developer, plate finisher, tobacco spit, deletion pens, acetone, diet soda, drain cleaner, or expensive whiskey will work instead of a proper scratch remover, they will give you deceiving result at best.
The Hone Test is best performed on press. Pick a spot in the background that will be easy to identify on the printed work. Hone (abrade, scrape, scratch, rub, whatever) a spot in the background, making sure you have physically removed the entire surface, right down to bare metal. Try to get this spot as smooth as you can. Now that you have a fresh surface to work with, you need to treat it to make it hydrophilic. This is done by rubbing it up with the scratch remover. The scratch remover will react with the exposed metal and form a fresh hydrophilic coating. The repair that you have made to the honed area is no match for the grain and coating applied by the plate manufacturer who uses heat and electricity as well as chemicals to do this and should not work as well as the plate background. Do not let scratch remover dry on the plate, rinse it off with water or fountain solution.
Start the press and when running, slowly reduce the dampening supply until the plate scums evenly and observe the result. If the honed area runs clean at any point when the background around it is scumming, the plate is sensitive. If the honed area scums at the same rate as the rest of the background, the plate is not sensitive.
If you observe the honed area running clean while the background around it scums, all you know at this point is that the plate background is sensitive; you do not have any information on why it is sensitive. But you do know that the problem is not the press, the ink, the fountain solution, the press settings, or your bad attitude, and you should then be able to focus your efforts on determining why the plate is sensitive. Likely explanations include, but are not limited to, old plates, fogged plates, plates that have been exposed to heat, dirty developer, poor rinsing, spent developer, you get the idea.
