Hello Rick, my entire layout is a winter scene as you can see from the attached photos.
I always used Matte Medium that I purchased from Scenery Express for areas that are close to where a person could blow or touch the snow. Little kids seem to enjoy blowing the snow more so than grown ups.
Must be a height thing !!!!
Further away areas I just sprinkled the snow on the scenery and built up areas with additional snow if needed.
Along with the Woodland Scenic Snow Product I also use Buffalo Snow Brand extra fine snow.
A bag of the Buffalo Snow Product will go a long way compared to a bottle of Woodland Scenic Snow.
Mix the two snows together as your putting in on your scenery to give the snow a different texture.
After the snow is down and dried (a few hours depending on your conditions) spray it again very lightly with Matte Medium and then add another light coat of snow.
I have not found anything so far that in time does not turn the snow a yellowish color.
Over the summer when my layout is covered up with plastic drop cloths it still gets dusty and the snow is not as white.
Simple fix, just like in the real world give your scenery a light dusting of snow and everything will look all fresh again.
Have fun !!!!!!