The biggest problem with urethane is that you have to lap the panels to join them. You get different expansion rates at the lapped areas when the area is in the sun and gets hot you will see the lapped area. Even welded and bogged lapped joints will do the same. Sorry if I upset anyone but I can't say I have ever seen windows filled in that don't look like crap after a few years especially if the van is driven. It's just such a flat in your face panel.

Cheers Dave