It says on the side of Wickes expanding polyurethane foam that, if used
externally, it should be covered (with filler or mortar) within 7 days since
it's not UV resistant.

Anyone have any experience of exactly *what* happens to this foam if it's
not covered within 7 days?

I've just braved the elements for 3 days moving an external door - the new
door is finally vaguely weatherproof (with a combination of expanding foam
and frame sealant) but I really can't be arsed rendering & painting around
it before the spring. Am I being foolish? If so, as a temporary measure,
could I just paint the foam with some form oil-based primer?

Ta!

Andy