Where would you normally use a treated timber wall plate?
As a decorative end support post, perhaps on a balcony or verandah.

What is it made out of?
Plantation grown Radiata Pine that has been H3 treated (suitable for above ground use).

How should you finish it?
A water based primer/undercoat is not a necessity (although it wouldn’t hurt), but at least 2 coats of the final colour is a must.
Leaving it unfinished, oiling it or using a clear lacquer is not recommended.
Speak to your local paint retailer for more info.