HP printers have the heads on each cartridge, thus is they cannot be freed by solvents, you need to replace the cartridge. Epson printers have the heads in the printer, therefore if you cannot free them by solvents, the heads need to be replaced. At £45 each for the head alone this is usually not economically viable. (last time I checked). The solvent, I was told, can be nail polish remover. But, get this checked first, I haven't tried it.