You should of course follow the directions that came with the product. The stuff I use is sprayed directly onto the platen, left to soak for 30 to 45 seconds, then wiped off. For the longest time I was using it on the work bench, indoors in an enclosed space, without any issues, but that was because I was mostly just using one application to clean the platen of a machine I happened to be working on. However, when I used it many times over on the rock hard platen it was a case of overexposure and I won't make that mistake again.

So this product you mentioned... Can it turn a rock hard platen into somewhat rubbery again?

Well I would include a link but apparently I need to have made two posts. : (
 

I have tried this on bail rollers and other small rollers (Not the platen!) succssessfully to make them soft. Get some tin foil, bunch it up, and rub it like sand paper against the roller. It may discolor the roller a tiny bit, but not very much. It will improve te grip of the roller greatly.

KatLondon wrote:

I use rubber rejuvenator on all my platens when /i first get the typewriters and give them a good thorough clean, and it does indeed improve the platen. If there's anything there to improve. I haven't seen it make any appreciable difference in a platen that is rock-hard and, eg, making holes in the paper when you type. 

You mean that's not normal? I just thought it was my over hard touch, LOL!  My periods invariably punch holes and my lowercase l's cut the paper, so I always use at least two 20 lb. backing sheets - more to protect the platen, than anything else. Being continually attacked with small punches cannot improve it.