I am working on a garage cabinet project and I used MDF as the material. I used a white primer for 2 coats and I like the result already. Can I leave it like this or I have to paint it with a proper paint? Basically can primer be used as paint?
There's no reason you can't leave it as-is as long as you are happy with the appearance. A lot of new homes in large developments don't have much more on the walls either.
You could have also used a primer/paint combo product and not have to apply any additional layers at all.
I don't think you'll have a problem with it. I built cabinets using mdf from Home Depot recently and was soaking small pieces of it in water for days to see what would happen to it and nothing ever did.