For acid etching of alumina you may possibly want to consider: hot H3PO4 conc. + HNO3 conc. (3:1 to 4:1 vol / vol); hot H3PO4 conc. + HCl conc.; warm H3PO4 conc. + HF conc.; hot H3PO4 conc. + CrO3*; warm to hot aqua regia; warm conc. HNO3 + conc. HF; warm conc. HF:

cf. http://onlinelibrary.wiley.com/doi/10.1111/j.1151-2916.1999.tb02279.x/abstract

(*) 4 g CrO3:7 cm3 conc. acid, or saturated sol.; 1 mol of potassium dichromate in presence of at least 1:2 mol/mol of phosphoric acid can be taken as equivalent to 2 mol of chromic acid referred to as CrO3: K2Cr2O7 + 2H3PO4 → 2CrO3 + 2KH2PO4 + H2O.