I’ve got a couple of thin, lightweight ‘summer’ waterproof jackets that I like to use in mild, wet weather (just like today in fact). On today’s ride, however, it became obvious that one of these wets out almost instantly and is in no way waterproof.

I assume reproofing is just as it is for a more substantial jacket – nikwax or Grangers spray on reproofer?

Nicolas Tec wash and then reproof with the nikiwax wash in stuff. I have been known to aerosol proof over the high traffic bits like shoulders not that it makes a huge difference