If the "varnish" is an acrylic, lacquer thinner will remove it. If it is varnish or polyurethane, it is much harder to remove. Try lacquer thinner. If that doesn't work you may need a harsher chemical like Marine or Aircraft paint remover, which contain methylene chloride (aka dichloromethane). NB: Lacquer thinner will harm styrene-like plastics, methylene chloride will harm a larger range of plastics. So, be careful, if you have connectors or devices with optical windows that might be harmed. Epoxy, like is used for IC chips, is relatively unaffected by both solvents mentioned.
John