Neaten up window connection

[Baskavoda]

I have renovated the living room and added a layer of renovation plasterboard on one wall. The existing frame remains and looks nice, but I'm a bit unsure about how to handle the bottom part.

How do I best solve the joint between the frame and the plasterboard?

Simply fill with wood filler and paint? Or is there a better method? There will be a windowsill on the outside with a small gap for air from the radiator below.

 

[Needas]

I've renovated the living room and there's a layer of renovation plasterboard on one wall. The existing casing remains and looks nice, but I'm a bit unsure how to finish the bottom.

How do I best solve the seam between the frame and the plasterboard?

Just spackle with wood filler and paint? Or is there a better method? There will be a window sill on top with a bit of space for air from the radiator below.

I would have made a windowsill in MDF and extended it a maximum of 20mm beyond the wall surface.
You can glue it on to avoid the screw heads.

 

[Needas]

After actually reading more carefully, I changed my mind and would just glue a strip of thin MDF to align with the outside plaster, then put on the windowsill.

 

[Baskavoda]

Use a moderately thin MDF, like 6mm.

But then the joint will end up between the MDF and plasterboard at the front edge instead? But maybe it's easier to hide anyway.

 

[hanbch]

I have renovated the living room and there is a layer of renovation gypsum on one wall. The existing molding remains and looks nice, but I'm a bit unsure about how to handle the bottom part.

How do I best solve the gap between the frame and the gypsum?

Just apply wood filler and paint? Or is there a better method? There will be a window ledge on top with some space for air from the radiator below.

 

[piko]

Are you going there with a windowsill with loose brackets? If so, I would probably go with @hanbch's suggestion with molding and touch up the existing reveal with lacquer paint.

It looks like it could be quite tricky to get a custom-cut MDF board/reveal board there.