you can try electrodeposited photoresist to protect the sidewall.

Reference:

Carsten Christensen, Peter Kersten, Sascha Henke and Siebe Bouwstra, "Wafer
Through-Hole Interconnections with High Vertical Wiring Densities", IEEE
Transactions on Components, Packaging, and Manufacturing Technology--Part A,
Vol. 19, No. 4, December 1996

hope it helps!

joyce