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I've done it for clients. It works but then they find the sealant around the edges is not opaque enough. It never seems necessary and thick curtains would do the job.
Do you mean they later decide it's too dark?
The problem is our current blackout curtains don't seal against the window tightly enough and so light gets around the edges.
hugo7-
Not dark enough, i.e. there is still a very small amount of light that can show around the sides because the sealant between the u channel and the wall is not absorbing it all.
Personally I don't understand why the 90% reduction in light from daylight blinds despite the leakage at the sides is not enough, I don't really get why people want 100% darkness during daylight hours. Seems to be a new requirement. Each to their own though.
As you are asking about the practical problems achieving that though, in my experience people are rarely satisfied with the results and don't achieve 100% blackout without a lot of work.
Airhead
I've done it for clients. It works but then they find the sealant around the edges is not opaque enough. It never seems necessary and thick curtains would do the job.