I know this thread has skewed away from polishing hard rubber a few posts back, but polishing is a gritty subject and only a matter of degree away from grinding, sanding, and other bad sounding abrasive based processes bandied about therein. The nitty gritty is that most folks think that "polishing" is somehow different from these harsher sounding words, when actually it is only a matter of degree of grit. The Micro Mesh hobby kit is a good case for the whole array of true grit. When taking a scratch out of a smooth surface one is actually bring the surrounding surface down to the scratch. (Reminds me of an old joke about whether Preparation H actually shrinks anything or just swells the surrounding area). The Micro Mesh package comes with grits running from 1800 to 12000. The way it goes is to start with the coarsest grit you need to get the surfaces almost even, then move up grit by grit, 1800,2400,3200,3600,4000,6000,8000,12000. When you get done and did not skip a step the surface will be almost glass smooth. Anyway each and every one of those numbers is removing some material, just that when you get past 6000 the human eye can not see the micro abrasion and at 12,000 it is hard to see under magnification. Finish it all off with even finer liquid than those grits and Shazam! Beauty emerges!. Simichrome has 2 things that get to the job on hard rubber. Grit (about 6000, if memory serves) and Ammonia. Ammonia was the original curing agent for rubber back in the day and breaks down oxidized surface rubber easily. Too much ammonia, too much grit and you get that rippled ripple effect on those Watermans. In my hands ammonia breaks down red hard rubber faster than black for some reason.
This thread has slid into reblackening in recent posts too. You can polish a pen down to "newly exposed" dark(er) rubber but maybe to the point where the details get worn away. An alternative for pens that are already in bad shape and as has been said many times for pens that would otherwise never be used because they are perceived as being too ugly, is reblackening. At 32 microns carbon particles or finer no visible surface details need be lost.
The whole thing about this is that the black has to get put back into/and onto the old rubber to make it black again. As David points out it is the broken down oxidized, UV'ed rubber on the surface that has let the carbon black go and it is this very dull, microscopically uneven surface that permits at least one of the relatively accepted blackening methods to work so well. There are only 2 main ways to do this (apart from shoe polish (don't do that) and paint (really don't do that). One way is "molecularly" out of solution (G-10 for example) where the carbon comes out of Carbon Disulfide (carefully as this liquid is extremely toxic) molecularly into the rubber, or from carbon black at under 32 microns (not coal dust, puleese) and is carried into and onto the rubber by a vehicle that will place it there evenly and leave an even protective surface shell (PMBHRPPNo.9 does it that way). The placement agent in that case is diluted ammonia. Ammonia is soluble in water, and it does not take much ammonia to create the right surface conditions to let the carbon black settle into the nooks and crannies,even things out, adhere and stay put. There IS real science involved here, but even a commercial banker pen hobbyist can be taught chemistry, by a rubber chemist with 50 years in the industry without having to get an advanced degree in biochemistry. Ooh now I sound defensive - sorry.
Not meaning to be tooo self serving here either, but there just aren't many people available to comment who have actually worked closely with all of this) outside of the 900 or so do it yourselfers that have worked with it, but who probably are not sure they are qualified to post authoritatively. So I thought I should speak up.
Edited by Wahlnut, 05 April 2011 - 15:27.