Hi It is a longtime since I did any HUVEC work but as far as I remember a low gelling temp gelatin (and/or FN) is a requirement for primary cell isolation. For the cell lines some researchers do and don't coat see:
we used to coat our flasks and dishes with gelatin when growing HUVEC, but we did it at 1% in PBS. We left it for 1h at room temperature, then removed the excess gelatin and washed once with PBS before seeding the cells. Hope that helps!