Glad you like it, only took me 8 years to find it

. I think it works because its mono, there is room between the sets of bristles to allow movement. It doesnt seem to wear. The bristles will "cut in" to the tops, corners and sides. I donty do frames so have no idea how it works on frames. The bristles have "give and splay"
Some may pick it up and say thatl never work, try it once and reckon its cheap. They would be right, its about as cheap as you can go.
For plastics I use the tesco or any flocked.
There is however a suttle difference between all the supermarket brands.
Just found a coservatory finial brush for £1