If you had done a better search, you would have found this topic discussed a little more recently than this one.
Yes, there are Signing Bonus'.
No, they are not offered for all Trades.
No, they are not offered all the time to all applicants to these Trades.
What Trades have a Signing Bonus? A Trade that has a manning shortage may offer a Signing Bonus.
Does everyone entering one of these Trades get a Signing Bonus? Yes, if they meet the criteria of the Signing Bonus, which means that they are already holding qualifications required for that Trade. No one will get a Signing Bonus in one of these Trades, if they are not able to meet the criteria.
Does a Signing Bonus apply to a Trade forever? No. Once that Trade has met their quota, the Signing Bonus will disappear.
A Signing Bonus is a tool used to attract "fully qualified" people into a Trade that is short personnel.
Untrained people are not likely to be in a position to receive a Signing Bonus.