The popularity of the National Gallery's new East Building has put such a strain on the institution's guard staff that the Gallery has been forced to close part of its old west building.

"We're about 20 or 30 guards short," Gallery administrator Joseph English said yesterday. "The new building is so crowded that we're needing more manpower than we anticipated."

Consequently rooms containing some of the Gallery's most famous paintings - Old Masters and Italian Renaissance works - have been sealed off rather than have the priceless masterpieces go unwatched.

"We had to do it about a week ago," English said. "We started hiring new guards well in advance of the opening of the new building, but an awful lot of people originally hired as guards went on to be building employes, which pays a lot more."

After an initial training period, guards reach a GS-4 level that pays $8,900 annually. The Gallery currently employs about 200 guards, up from 120 before the new building opened.

"We've been running ads in The Post and The Star for two or three months," English said. "Part of the problem has been with the Civil Service Commission, which has just reopened its registers on the matter. Federal hiring is very regulated, and until now the fact that a person might be very surly or unable to communicate wasn't taken into consideration by them. It made our problem much more difficult."