Discussion about the Korg PolySix synthesizer

I repaired two intermittent keys on my P6 on the weekend using some used keys from a scrapped P61 keyboard. The used keys had been modified by filling the small plastic cavity directly above the key contact with some kind of hard resin. This resin added a tiny bit of weight but more importantly it changed how the key pushes onto the rubber key contact. In my case this dramatically improved the reliability of the key strike. It could be my imagination but the weight feels a little better when I play too. 

It got me thinking that adding something with a bit more weight might improve the feel and reliability of the key contacts. My Kurzweil K2000 has a much nicer action and I think its due to the weight.

Wondering whether anyone has seen a modification like this before...?

Anyone know what the resin is...?