The Corona 4 has a hard platen that will eventually be sent to JJ Short for resurfacing. In the interim I've decided to try various suggestions I've read online to gain first hand experience for future reference.

This is the sequence of the experiments.

1) The original platen was cleaned with soft scrub, sanded on the lathe with 320 sandpaper then rubbed with DOT 3 brake fluid which was left on the surface for about 3 hours. After drying overnight it went back in the machine. The result was a still hard platen with plenty of grip to easily load paper. Typing with 2 or three backing sheets worked pretty well. I'm new to typewriters so assumed the noise level was normal for a hard platen.

2) Avery 73601 was substituted for the 2 or 3 sheets of copy paper as a backing sheet. The resulting type was sharper and the noise was noticeably reduced. This is a pretty easy and very cheap improvement for a hard platen. I doubt it gives the full effect of a new platen but the results would satisfy me were it not for curiosity about the other options.

3) I turned about 0.050" off the diameter of the platen, quite easy when they are hard because it machines like PVC pipe at that point. The platen was covered with a section of bicycle inner tube turned inside out then put back in the lathe for sanding with 320 grit to get rid of the small bumps caused by mold ridges in the tube and to restore the original diameter. The result is a (too?) grippy platen that feels nice to the hand but as suspected is a bit too soft. Adjusting the paper is a challenge and the impression left with no backing sheet is a little thick in appearance. The noise level dropped considerably. Using 1 or 2 sheets of paper for a backer helps too. Personally I'd go with the Avery 73601 on a DOT 3 treated platen and forget about the inner tube. 

4) Next I'll remove the inner tube and replace it with a layer of heatshrink. For the sake of being thorough I may then try two layers of heatshrink. The next experiment would be one layer of inner tube covered by one layer of heatshrink. I might as well see what I like and what I don't as the biggest cost is my time. 

5) Concurrent with #4 is the most extreme test. I'm building a new platen using aluminum tube covered with PVC tubing, perhaps I'll call it the "Supercharged" platen. ;-)  I don't want to strip the original platen down to the core just yet to "Turbo" it so this seemed like the next best option. The platen on the Corona 4 is a pretty simple thing to replicate. I'm quite curious to see how the result performs. I've got the PVC installed on the aluminum core and right end plug is finished. Tomorrow I'll be able to do some test typing on the surface as the only feature missing will be line spacing. I'll turn the platen by hand to go to the next line. It won't be consistent but I'll be able to see how it grips, what the type look like, how it feels and whether it is quiet. I won't be able to measure the return speed of the typebars though. ;-) I don't own a durometer so I won't know the hardness. I can tell you it is considerably softer than the original. It also looks pretty cool. Doing the same thing with a wood core would be equally simple and might offer some different properties.

Thanks for reading.

Brent