never done a sinew bow, but have read that combing the dry sinew very well before soaking- really helps get a smooth finish.
it was combed with a block of wood with a bunch of fine( finishing) nails through the block- geuss one should pre drill the holes for the nails, or all trhose nails will split the wood.
the comb was clamped to the table and the sinew was pulled through it.