Hi Helen
Honed and filled travertine has a high surface tension and so sealing isnt always required depends a little on the quallity. Either way it doesnt absorb much so gently gently with it or it will sit on the surface and harden off or chrystalise.
Your man at topps goes on what he is told and as the sealing manufacturers are kept afloat by the "it cost a lot so needs protecting" mentality he will recomend it.
Dust on tiles (hoover), rollers cause bubbles, brushes streak all can give a rough feel and appearance to honed trav which has a good finish.
The topps man is correct in small part by saying use more as this will soften the existing sealer but its a little fiddley and could backfire on you.
Try as I have said above remembering you are trying to remove the grainyness you have.
tiler
..