Extending this surface beyond purple solid how?

Hi,
V5
I am struggling to know how to finish off this surface for successfull boolean difference.
( I had it as a solid but have noew removed the capping surface to see whats going on.

It feathers out at downturn, how can I extend it all to run clear of the purple surface ?
main part can extendSrf downwards, but it wont let me do the entire edge, only in stages, so trim it to a standard shape methinks,…but then…that last part I dont want to extend downwards but outwards.

I tried dupedge and offset intending to sweep2 but get an oddment in that feather area, and now struggle there.
Boolean Difference how to make good this shape.3dm (776.8 KB)

Steve

Hi Steve - I would extract these faces

image

and deal with it at the surface level, then stick it all back together.

However, this object has some skinny, zero area faces you should get rid of:

image

Then, DupBorder on the red thing, use these curves and the brep itself to trim up the extracted faces and vice-versa, then Join
image

image

image

image

-Pascal

Hi Pascal,
now done, given the feather out a bit more curve, and trimmed surfaces and the item with surface, joined and all is good.
Purple solid gulley solved

Purple solid gulley solved.3dm (753.6 KB)

many Thanks.

Cheers

Steve