Had to run off and watch Survivor China with the Family.We're addicts.About the only show we all watch together.
Anyways,I have done some window trim around aluminum clad windows that calls for a little extra thought.I tried it out about 25 years ago ,and it still looks good.I had my doubts when I first did it ,but it seemed to make sense at the time.
It's was a lot of work then,but it looked great.I would rip down 1/2" plywood in 2 1/2 " strips( Double it up so it was 1" deep) .Then nail these around the perimeter of the window,leaving it 1/8" from the window frame.When I bent the coil I would put a hem on the side toward the window(usually a 1" bend with a !/2" hem),being careful not to fully close it.The J would be formed into the other side.Then take a long straight edge,like another ply-rip ,on edge, and tap it into the gap between the window and the 2 1/2" strips.Make sure it's cut before you install it cause you won't pull it out.
Of course that was before I started using the 3 1/2" lineal.which is a lot faster,though expensive.
To match new windows to old ,I would build a sill out of 2x4's, ripped with a bevel for pitch ,for the window bottoms.
Using the same method as the sides,I would hem the back of the sill piece and simply bend it up flat against the wall where it passed the window sides to shed water from the J-section of the window trim over the top of the sill.
It's probably hard to picture exactly what I 'm trying to explain,but in certain locations it may come in handy.