Homebrew PCBs

I have this RTV silicone meant for acrylic glass, it says on the packaging  
that it is good for polycarbonate.

Well, it doesn't cure. After opening it i found it looking dull and all  
strange, i actually made sure twice that it is silicone and not acrylic  
sealant.

After trying on a piece of lexan it doesn't cure at all (5 hours).

A sample of standard silicone shows a skin in a few minutes.

The manufactuing date is 11/2000.

What is wrong with it? is it too old? I have never encountered this  
problem, usually it cures inside the container.
I'm glad i did not use it without trying it first.

Please let me know if you know what is going on, and also if you happen to  
know if there is substantial difference in adhesion between silicone meant  
for acrylic glass and standard silicone.

thanks

ST