I've made red wine from juice for about 4 years now using various grapes; Merlot, Pinot Noir, Montepoluciano, Malbec and have recently found staining on the inside where the bottle was laying on it's side. This is not happening on all the bottles just a few. The grape is Pinot Noir. What is causing this? The wine tasts great. I let the wine ferment in carboys for approx. 10-12 months before botteling. I have used Potassium Mibeta sulfate to stop the fermentation prior to botteling.