The Bombarding is the process in which the neon unit is heated up and at the same time the vacuum pump extracts all the air out of the unit.So the reason for this stage is to purify the inner walls of the neon tube and create a vacuum . Impurities inside the tube would cause the neon to only light up for a very short time .
The parts of the bombarding equipment are ;

The high voltage transformer ………(most of the time it’s a 10 ,15, or 22 kva tranformer controlled by a choke or variac)
The vacuum Pump …………………….( one of the most used is the welch 1402 belt driven vacuum pump)
The Manifold System ………………….(The best one is the O ring manifold system)

At a certain vacuum the air inside the glass will conduct electricity , electricity that is produced by the high voltage transformer, and will at the same time due to the high voltage and amperage used will heat up the glass , heat that is needed to purify the inner walls of the glass in the neon sign . At the same time the vacuum pump will extract the burned impurities that have been burned by the heat produced. After there is no indication in the gauge that is monitoring the pressure that there is no more increase in pressure caused by the burning of the impurities and the electrodes have reached a orange red hot glow , then the electricity is turned off and the unit is left to cool down .When the unit is cooled to room temperature then the vacuum pump valve is closed and the gas valve is opened to introduce into the neon unit a certain amount of gas . Then the unit is sealed by means of a hand torch and if it’s a mercury unit the mercury is transferred from the temporary bubble deposit into the neon unit then the excess tubulation with bubble is separated from the neon unit and discarded  using the hand torch.

Here’s a short video of the bending and bombarding of a neon unit