Smoothing masonite joins

[elphingirl]

Hi all

I am hoping someone can suggest what might be the best material for smoothing off two pieces of masonite joined at right angles (like an external plaster join). The masonite is smooth side out, and is pre-primed. I was thinking Polyfiller, but it is in an area where it might get damaged easily, so I was wondering whether I could attached some metal plaster corner, and use plaster top cost.
I also have the situation where the masonite meets plaster surface too.
Any comments appreciated, cheers
Justine

[aussiecolector]

I have always used a cover strip over versalux on external corners. It may be possible but I'm no plasterer.