Just two comments (but there must be many readers more qualified to comment than myself):
(1) re "shot sitting in the line blocking the suction effect" - you must keep running air for a few seconds after switching off the shot feed in order to clear the line. If this does not work then you appear to have supply pipe problems (such as air-intake blockage).
(2) Increasing the nozzle diameter is the obvious answer to your coverage problem provided that your air supply (compressor, ballast tank and pipe diameters) can cope. Doubling the diameter would then reduce coverage times by approximately a factor of four.
