Think you might find that the 'film' is either varnish or a lacquer coating from the factory.
If you are wanting to fix it, you will probably have to use varnish and a brush... that said you can buy aerosol lacquers.
The window will need rubbing down first with sandpaper and you need to check for rot and treat this (if there is any).
You might be able to get some of the black staining out of the wood with oxalic acid... it might be better to paint the window.
You might need to check the room ventilation as this could be the cause of the black... or a leak.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local