walnut shells pecan shells corn husks peach pits etc typically the surface abrasion removes about 0 01 0 04 preferably 0 025 0 035 inch of the board surface in producing the desired look depending on factors such as the type of abrasive and board being treated as well as the spatial relationship of the wheels and board the wheels can be operated at various speeds such as from about 1800 to 3600 rpm abrasive granules thrown by the first discharging means against the upper surface of board 15 rebound therefrom and collect inside cabinet 20 and disadvantageously on the board surface if this spent blast medium is not removed from the surface of the board prior to the surface 39 s exposure to another discharging means an unacceptable condition known as 34 masking 34 occurs this phenomenon results from blast medium covering the surface of the board and then being hit by subsequently thrown blast medium the effect is lower relative abrasion from the second discharging means and even when abrasion is evident the character of the abrasion changes such that the texture of the masked area appears different from surrounding areas when painted presenting an undesirably variable appearance a blower means disposed between the first and second abrasive discharging means removes from the board surface the spent abrasive particles thrown by the former means before exposure of the surface to the second abrasive discharging means the blower means advantageously comprises a roof mounted device which supplies air under pressure to blow the spent abrasive off the board surface as illustrated in fig2 to 4 air under pressure is directed from a suitable source not shown to a manifold 24 whereby air is.