Not saying this is the best stuff, but I've just covered the inside of the arms I fitted a few months ago with this
https://www.ebay.co.uk/itm/3-x-Dinit...MAAOSwSFVboR4J
Did the arms and sills, for under £40, seemed ok.
Used this to cover the outside as they appear poorly protected in the outside as well
https://www.ebay.co.uk/itm/DINITROL-...MAAOSwmudaCvMg
I am not changing them again, so will treat them every year or two to make sure they last.
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