Crestar Financial Corp. yesterday became the second major Virginia bank this week to boost its loan reserves to cover possible losses on Third World loans.

The Richmond-based bank holding company announced an additional $10.5 million provision for Latin American loans in its fourth quarter, bringing to $28.5 million the amount it added to loss reserves in 1987 to cover loans to those countries and unallocated reserves.

Crestar's reserves against foreign loans now are almost 60 percent, meaning the bank holding company expects to recover only about two-fifths the value of those loans.

Signet Bank, also of Richmond, announced similar steps this week, as did many regional banks nationwide.

One result of these actions was a 31 percent drop in Crestar's earnings during the year, to $56.7 million ($1.92 per share), compared to $82.6 million ($2.89) in 1986.

Net earnings dropped 39 percent in the fourth quarter, to $12 million (40 cents) from $19.6 million (68 cents) the year before.

"The additional provision in the fourth quarter was consistent with the actions taken by other strongly capitalized regional banks to deal with this longstanding problem," Crestar Chairman Richard G. Tilghman said in a statement.

He added, "We believe we are sufficiently reserved to deal effectively with the problem."

John Hanson Savings Bank of Beltsville reported net income of $1.22 million (20 cents) for its second quarter ended Dec. 31, up 6 percent from $1.16 million (20 cents) in the same period a year ago. For the first six months of the year, profits tripled from $789,000 (14 cents) to $2.12 million (35 cents).

As of Dec. 31, John Hanson reported assets of $1.11 billion, up 37 percent from $807 million a year ago.