I need to make perfect films. I have a lot of Formvar in Ethylene Dichloride, but it has been on the shelf unopened since 1990. As long as you keep it in the dark, it can last a long time. When exposed to light, you tend to form HCl molecules in the dichlorethane weakening the formvar.
Formvar solutions deterioate due to atmospheric moisture and age.
This causes holes. It is not worth ruining sections because one has used old Formvar solution. Buy or make new from powder. Make a few films and check in the TEM for stability, thickness, etc., before making a thousand. I am using the solution at 1 year and obtained excellent films.