When I first learned to quilt we were taught to use the sizing .... we can't buy it here in Canada any longer!
I always think of it as a weak starch, and I do not know the technical difference, so interesting to read what the others have said.
What I always liked about it is that it does not flake, like the starch does. Interesting that you have said to not iron immediately .... how long do you let it sit and soak before pressing?
Mary Ellen's Best Press must have the border sealed up, as now the spray starch is becoming hard to find. And MEBP is just too costly here, so I've avoided it.