In many ways, your arco technique will develop in relationship to your string choice. I began on spiros, went through school and changed strings at my teacher’s request, always felt lacking in the arco department, went back to Spiros, and much of my deficiency was alleviated. Find a tone you like then play the arco practice game and improve your technique with everyday practice. I love Spiros and can play everything arco on them within my ability. I play excerpts, solo, chamber, improv, folk, and jazz on them with equal facility. My 2¢.