You are better putting weedkiller down at least a week before you clean,if not after cleaning but before sanding.You seal after sanding to harden the joints but make sure you have had a good period of sunny weather and all the damp is out or you will have a headache.I sealed one last year,gave it a quick first coat of sealer,then sanded it as the sand flew across the smooth surface and down the joints,i then gave it another 2 coats of sealer.If i were you i would not start sealing till we hit the summer and the ground is totally dried out or you will be back in a month trying to get the bloom out.Good luck,Dan