DIY gone wrong (closing off old door)

Hi

I am new to this forum and DIY in general. We just purchased a unit so i started doing some DIY around the place.
This forum has helped me with heaps of info but now I am stuck.

I am trying to close off second bedroom door with some plaster boards. Door opening is 2100x900 and depth is 150 mm. The wall is brick coated with render and painted. I have created a frame(pine) and put the plaster board up.I have also tried to set the joints( I am using Gyprock multi purpose joint cement)
Ok there is a few problems I got

-Depth is 150 mm I could not find timber that would give me 10 mm gap on both sides.
-After setting joints (first coat) the finish rough espetialy where tape meets old wall. Can I sand before applying second coat. Should I start from beginning or get a pro.
I have been quoted $400. Any help would be great

Regards

Edvin

If anyone know good plasterer in Lane Cove area would be great
.


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