How to convert a dry-laid flagstone patio to wet-laid?

You can't convert without tearing up the whole patio. To "wet lay" a patio, a 4" slab of concrete must be poured, then each individual stone is hand set. This is an extremely labor intensive and expensive project, and not recommended for the do-it-yourselfer.

To alleviate the weeds growing between your patio stones, scrape out what ever is in there now down 2 or 3 inches. Replace the joints with stone dust. Stone dust is completely inert so weeds will not grow in it.

The slabs will have been laid on a compacted sand bed, I would advise that you carefully take up the patio and with a long lath or other piece of wood, gently level the sand and compact it by walking on it. Don't disturb the sand too deep, only about half an inch or so. now you have done this lay over the sand a weed suppressant fabric, that is a black sheeting that will allow water to drain through but keep the weeds down. Now re lay the patio slabs and relax. this is only a short and simple suggestion for a not too badly uneven patio.