I have 6 horses pastured together. None are new to me. One mare is losing ALL her hair. Why???The skin is perfectly smooth. No bumps or abrasions. 

You might check out a product called Coat Defense.

I have an older gelding who is always the last to shed. He gets bald patches pretty often. 

My 19 yr old molts, not sheds, every year. I've owned her for 18 years and its what she does. Right now she is bald in most spots. Give her a week or 2 and her summer coat will be in. I have had to put a sheet on her the last 4 nights since its dropped into the 30's but shes none the worse for it. My other 3 look awesome and are shedding normal. She is the oddball. 

I never thought of molting!!!   But C's black mare "molts" in patches this time of year. . . . I always thought it was just her and so treat her each spring with medicated-smells-wonderful skin care product.   

Some horses shed earlier and shed a ton more out vs others. Things to consider is: climate, when the sun rises/sets compared to 1-2 months ago, and quality of feed. Make sure your hay is sourced responsibly and is good quality, and if you have the horse on a grain supplement, to make sure it's formulated to meet her physical needs (senior, performance horse, etc)

I would be more concerned if she was rapidly blowing out her coat. But for some, like my mare, shed the second sunset is cut a minute shoot and boom- no more winter coat in a week.

I was thinking about it and she started this way of shedding about 9 years ago. She had a bad eye injury and was on banamine and 4 different eye drops a day. Banamine am & pm and drops 4-5 times a day initially. We treated the eye for 4 months. I put 5 bottles of banamine through her. That started in Feb and we treated until May I think it was. She actually lost 90% of her hair. The vet said a fungus. I disagreed and chalked it up to all the banamine and drugs we had pumped in her for months. Saved her eye and she can see out of it. Ever since then, she starts letting go of winter hair roughly 3-4 weeks after my others start. And when she does, its all coming out. Not brush and get some off, repeat the next day for a week. She literally loses clumps of hair all over her body and you can see her skin. I was petting on her this morning when I fed, in the dark. And she is velvety where the new hair has grown in. Every day she looks less mangy and more normal. Its just her. 

I'm giving up. New hair is growing in. I don't understand why she "molted" this year. 

I have one of those too.  She does it every year.  She goes completely bald and then has the shiniest 'copper penny' glisten, softest coat of any of the others.  Her coat stays shiny and soft all year too.  I quit worrying about it.  She doesn't itch, she's not sensitive and her skin is dark. She doesn't care that she's naked so I don't either.  

Oh, and she honestly has the thickest, longest tail I've ever seen on a horse.  Her mane is to the point of her shoulder impressive but her tail....  better than the fake, show horse tails you can buy.

I had a mare once that always shed to the skin, and she did it every year.