Which of the following is used for etching porcelain?
1.23% HF
4% HF
9.6% HF
37% HF
Chemical preparation of glazed ceramic may be done with 9.6% hydrofluoric acid (HFA) or acidulated phosphate fluoride and then treated with silane coupling agent to increase the bond strength.
Zachrisson recommends the following techniques for bonding to porcelain surfaces:

Isolation is very critical in this technique, not just to prevent contamination with saliva and water, but also to prevent soft tissues contact with HFA.
Barrier gel such as Kool-Dam prevents flowing of etchant onto the gingival tissue.
Deglaze the area slightly larger than bracket base by sandblasting with 50 µm aluminium oxide for 3 s.
Etch porcelain with 9.6% HF acid for 2 min.
Carefully remove gel with cotton roll and then rinse using high volume suction.
Dry with air, apply silane coupling agent and bond bracket conventionally.