A Sydney man who raped a woman in front of her four-year-old son before igniting the gunpowder he had poured into her eyes, has been sentenced to at least 18 years in prison.

Mark James Dennis will serve up to 24 years in prison, after being found guilty of 13 sex and robbery charges, including rape and indecent assault.

Dennis, 38, of Tregear, in Sydney's west, admitted abducting and sexually assaulting a woman and a schoolgirl at knifepoint in separate attacks in Sydney's south-west in 2003 and 2005.