Hey berntd,
Your description of blistering and volcano appearance indicate moisture and vaporing in the metal along with perhaps dirt residue. The oxy/acet. application and remelting has heated the metal to dissipate this moisture. Brown deposits indicate dirt and rust forming on the welded area. Your statement of getting "reasonable welds" should also tell you that you need to do some serious practicing and learn welding techniques to improve your skills. You must make a mindset that "reasonable" is not acceptable...."excellent" is acceptable. Consider some schooling in a Vo-Tech school, college class, or even a high school class to enhance your welding education and proficiency. With experience, you will find the answers to problematic issues as this and will be better prepared to understand welding parameters to avoid these conditions. Learning will not be acquired overnight....it takes work, practice, and the thought that you will learn and get better with each mistake....there is no room for frustration and anger....simply walk away, think, and start another time.....Denny