You always see folks driving cars around with primer on them (or totally
covered with primer).  However I have heard that primer is pourous so that
paint will stick to it.  And water will soak right into it and then it will
hold the water to the body of the vehicle, actually making your rust problem
worse.  A very good idea (as Gary already mentioned) is to paint over the
primer, even with some shaky can paint, as a temporary thing until you can
afford to have the whole van painted.  It will seal up the primer against
moisture, and you get to allow your artistic side to shine through (for me it
is merely a flicker).  If you are really particular you can still get cans of
touch-up paint from the dealer for not too much money that matches your paint
exactly.
Or you can just smear daubs of orange rust-inhibitor all over your van (Chris
Turner knows who I am talking about- a former List member). :)
Ken Wilford
Van-Again
John 3:16