IT’S A FAIR GUESS that companies based in Virginia launch thousands of new products each year. Precious few of them get to mark the event with a luncheon hosted by Gov. Robert F. McDonnell (R) at the Executive Mansion in Richmond or are treated to a personal plug by his wife.

A notable exception is Star Scientific Inc., based in the Richmond suburbs, whose chief executive, Jonnie R. Williams Sr., has lavished tens of thousands of dollars in cash and gifts on Mr. McDonnell and his political action committee since before he became governor.

Under Virginia’s financial disclosure laws, politicians such as Mr. McDonnell may accept cash and gifts in virtually any amount; they must simply disclose any donation of more than $50. The idea is that full transparency will act as a brake on largess that looks too much like outright bribery. But in the case of Mr. McDonnell and his benefactor Mr. Williams, even those extravagantly permissive laws proved too onerous.

As The Post’s Rosalind S. Helderman reported , Mr. McDonnell failed to report a $15,000 gift from Mr. Williams that covered most of a catering bill for food and flowers at his daughter Cailin’s wedding nearly two years ago. He also issued a misleading statement about the nature of the gift.

When first asked about the $15,000 check, Mr. McDonnell said he didn’t disclose it because it was a gift to his daughter for her wedding. Astonishingly, gifts to politicians’ family members are exempted from the state disclosure requirements, a gargantuan loophole.

Documents subsequently obtained by The Post show that Mr. Williams’s “gift” covered about three-quarters of a $19,400 catering contract that Mr. McDonnell himself signed, annotated and started to pay, to the tune of $8,000 in deposits. About two weeks before the June 2011 wedding, Mr. Williams stepped in with his $15,000 check to cover the balance due — and more.

When the caterer sent a refund for the overpayment, the check was made out to Maureen McDonnell, the governor’s wife, who, three days before the wedding, flew to Florida to promote a key product being rolled out by Mr. Williams’s company.

Any reasonable person looking at those facts would conclude that Mr. Williams’s check was a gift to the governor. Mr. McDonnell, a lawyer renowned for his attention to detail, was astute enough to amend the catering contract to insist on a refund if the wedding reception was canceled due to an act of God or death. He was presumably also astute enough to remember that he had signed the contract and was on the hook for the balance.

Have other gifts been directed to the governor’s family? Mr. McDonnell, taking cover behind the disclosure law’s loophole, isn’t saying.

That’s a miscalculation. If he wants to get in front of what is rapidly becoming a scandal that threatens to engulf his last year in office, Mr. McDonnell should come clean with more answers than he has provided so far.