Fitting architraves to uneven window frames

[peter_sm]

The timber frame for the double hung windows is about 12mm proud of the wall framework inside.

I have lined the internal wall with 4.5mm cement sheet, and of course this now means the frame of the window is still about 8mm proud.

I cannot plane the frames back, as the internal bead that holds the hung windows in sits almost level with it, maybe pretty difficult to do.

I want to fit any architrave, it does not have to be bullnose. Could I rebate the architrave 8mm on one half of the back side to make it fit flusherer to the cement sheet on the outside? Do you think it would look silly? I may have to use just straight rectangular MDF preprimed 68 x 19mm or so?

[echnidna]

you can rebate the arc or

make some 8mm packing strip that fits against the cement sheet and the arc will sit on the strip.
or

if there are minimal gaps you could use 12mm quad at the edge of the window

[les88]

What I used to do is use a piece of 50mm X 25mm rip it in half so that it is square. Hold it against the frame and mark it with a pencil. Rip / plane to the line then glue it to the architrave. By doing this way it doesn't matter if the wall and frame are not parallel to each other.
les