I keep hearing that after 8-10 coats of Zaino, you need to reapply Z1 or use zfx.

Why?

 

Both Z1 and ZFX act as a bonding agent to strenghten the bond that the polish (Z2, Z3, or Z5) makes with the paint. It's kind of like a 2-part epoxy system. You need both parts to ensure a strong bond, better durability, more gloss, etc.... After 8-10 coats, using Z1 or ZFX, tightens up the bond again, improves gloss and slickness, etc.

You've seen those 2-part superglues that require two separate chemicals that interact to form the extreme bond. Even though Zaino isn't anything like glue, it functions similarly chemically in that the two parts mixed together are greater than either used individually. Kind of like synergy, where 1+1=3