You get a “clean slate” if your coming into the Catholic Church (or any Christian community, for that matter) includes being baptized (Important: baptism is a non-repeatable, once-in-a-lifetime event).
Since Newt was presumably already baptized, he did not get a clean slate by virtue of being baptized when he became a Catholic. He did get a clean slate by virtue of receiving the Sacrament of Penance at that time, but his ability to do so was contingent on his two prior marriages being annulled. I have no information about the grounds for annulment in either case, but have no trouble believing that valid reasons did exist. I accept his conversion as genuine.
The fact that Newt is now a Catholic with access to the sacraments pleases me personally, but has no bearing on my interest in supporting him politically. It certainly does not make him a saint. I think Newt’s a phony, a windbag, a prodigious ego, and not to be trusted. I disagree with his support of torture. I am not aware that he’s ever renounced past statements he’s made in favor of embryonic stem cell research. I will not be voting for him in either the primary or general election.