It's difficult to learn in 5 days. Plastering is a trade where you learn from your mistakes, there is so much that can't be taught. You literally have to **** things up before you improve, as mad as that sounds.
Those courses are OK to teach you the basics
But just the basics
And that's it
I did a wall with an adhesive taping compound once
It was a straight flat wall that was thankfully going to be papered over