Removing gloss paint from exterior window sills?

Hi everyone, my exterior window sills are painted in gloss. I want to remove it and apply exterior emulsion instead, as I'm getiing an ingress of water and I believe the gloss is to blame. Firstly, do you agree that the gloss could be the problem? I reckon the rain is getting in via hairline cracks and cannot escape. Secondly, what would you recommend to remove the gloss (I have UPVC windows, so need to be careful). Thirdly, how should I prepare the surface before applying the masonary paint? I ume I need to wash of all residue thoroughly, but would I need to use stabilising solution before painting? Thanks.