I have a thin white film on top of wine in the carboy? What ...

I have a thin white film on top of wine in the carboy? What is it? Is it bad?

Might just be from the yeast. Is it fermenting at this point or aging or what? What kind of wine is it?

We pressed about 3 weeks ago, added potassium metabisulfate and it is in the carboy with airlock. A few days ago we noticed the film. There is about a half inch of lees in the bottom of the carboy. There is no strange odor to it. It smells grapey.

Ok I am going to make 2 assumptions, one is that you crushed grapes and did a primary fermentation, the second is that you did transfer to the carboy and finished the fermentation (secondary fermentation) in there under an airlock before you added the sulfite.

Sometimes you get a yeast film on the wine. The fact that you added sulfite most likely means that anything else in there is not going to grow IE mold etc. If we can get OZ to weigh in on this we might get a more concise answer but I am pretty sure you are ok, but you might want to rack it off the lees just in case something funky is going on. I'll PM OzWino for you.

It sounds like you've got mycoderma floating on the surface. This is basically a mixture aerobic bacteria/fungus/yeast that will eventually spoil your wine if you don't do something about it quickly! The microorganisms in the film will pretty rapidly oxidise ethanol into acetaldehyde. Vinegar producers love the stuff but it's really something we want to avoid in wines. It's pretty rare to have it occur during winemaking, but a combo of various factors can result in favourable conditions for its development (eg. high dissolved oxygen, high pH, low SO2)

Sounds like you have caught it early in it's development, so no harm done... YET.

If the film is near the top of a carboy, I think the best method is to scoop it with a spoon and then wipe the edges with a clean cloth. or you could try using a syringe to suck out the film. I would be hesitant in recommending a racking, as the film breaks up easily and will sink to the bottom, making it impossible to get out without filtration (which is obviously an option you could consider).

Once you have removed it make sure the carboy is topped and spray the surface liberally with a 10% sulfite solution. Ensure you maintain your free SO2 levels in the wine to about 40-45ppm which is higher than normal, but in your case recommended

It could also be air mold because there is air getting into the container. To check whether or not the wine is ruined, take a swig. If it does not taste good, spit it out.