I've read that apple cider vinegar is a natural antifungal, and has been useful for people struggling with dandruff.
So first question, yes acetic acid in vinegar is known to be antifungal and antibacterial (J Food Prot, pg 2404-2409, 2008 and Commun Agric Appl Biol Sci, pg 265-71, 2008 ). As with all natural products there will be variances in how much acetic acid it contains and the antifungal/antibacterial activity also varies as a result.
It may help if your dandruff is as a result of a fungal infection. However, my advice is if you have dandruff and have tried the commercial anti-dandruff shampoos without any results, first get a diagnosis from a doctor.
How should I apply it? Should I dilute it with water? Let it sit on the scalp? How often should it be used? Will it damage my hair at all?
I haven't found a scientific answer to this question. There is research that says women prior to the manufacture of proper shampoos did use vinegar as a final acid rinse to remove limescale (Journal of the Society of Cosmetic Chemists, pg116-122, 1958).
As no one has published how often or how much to use, I can't answer the question....sorry!
Final additional question from me - Can an ACV rinse close my cuticles to stop porosity?
I have to admit I am guilty of thinking of the hair cuticle in this way. However, after setting up this blog and doing the research I have to come to the conclusion that the hair cuticle just doesn't behave like this. It cannot just be opened and closed in a simple way.
I have not found research specific to ACV, but certainly to acid conditions. ACV is a weak-ish acid and has a pH of about 3 (mine from my cupboard). Research shows that water absorption and surface roughness of hair appears to be quite similar at pH3 (acid) and pH7 (water). It actually only appears to change at high pH values (like for example relaxers at pH9/10). (J Invest Dermatol, pg 96-99, 1995 and Scanning, pg 431-437,1997).
If the cuticle was 'closed' at low pH, then water absorption should be lower and the surface roughness should change substantially. These two factors do not change, therefore the cuticle has not undergone a dramatic change. I therefore would think that an ACV rinse would not be able to drammatically change hair porosity.
Do you use ACV, if so how much/how often, what are your results?