Y
Yorkie
Dependant upon which natural your laying it is advisable too seal BEFORE laying and before using a wet cutter as this can cause stainage although as mentioned you can lay and seal if you keep yourself clean, but you run the risk of something staining it and if your only going to seal it anyway you may as well do them all together in the first place beforehand to be on the safe side but its "horses for courses" on that one. When laying slate its ok to lay then clean them by giving them a quick wipe over before sealing checking that theres no footprints etc as you don't want a size nine reebok classic sealed in the centre, grout then seal again.