It is just me or my Tamiya cement is drying up? I remember the day I bought it, it was as fluid as water, now, it had became thick as ketchup. I know I can buy another one easily, but I was wondering how to extend the lifespan of cements? Or, is there a way to dilute the thick cement?

Thanks!

The really thin ones have the Habit of some of the Chemicals Evaporating it seems no matter how tight the lid is on it still happens.........

I've noticed that myself and I thought I was a bit nuts thinking that! (Well, I am, but that's another story......)
Mine isn't like ketchup, but it is thicker then when I first got it. I had put it down to some of the plastic getting in there from the brush as transfer, just a small microscopic amount each time, but building over time and not to any evaporation.

I have managed to thin some down so i could fill a small seam that ran down the inside of an engine intake i mixed some of hannants quick dry thinners into it (think ordinary thinners might work aswell) I let it run into place and once it had dried the seam was no longer visible. Hope this helps.