Originally Posted by
Dave Demaree
1. The anodic porous structure is in poor condition due to dissolution or other reasons, causing the pores to not fully close during the sealing phase. As a result, the dye is slowly leaching out, darkening the bleached areas in the process. However, when this happens, the part normally has issues accepting the dye at all, as the pores will either be too small or too large.
2. The parts weren't properly sealed, again causing the dye to leach out of the pore structure.