obviously just be carefull around the edges or anywhere which is damaged, start in a out of the way corner and just check its not damaging aything. I never have and i have done some pretty old ones. If it has any white stones in it, they will come out and it can be noticable, so the lower pressure the better.
Tbh, take a bit of common sense with you and you will haev no probs, If you dont have any try ebay, thats where i get mine from.
![Grin ;D](http://www.cleanitup.co.uk/smf/Smileys/classic/grin.gif)