Hi Emily,
Not sure what a cistern is but you might try sodium silicate. this has another name, they call it waterglass, I have seen it used to seal engine blocks of porosity and it worked. Google it for more information.
Joseppie
November 2008
I can answer this question...
I have a hairline crack in the cistern, water is slowly leaking out. ?
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix toilets.