Jane, this is something I have used with good results. It says it holds for 3 hours, not sure if it would for a longer time, perhaps you could test it. If you do, please post the results. It works on crackers or bread.

CRACKER SEALER

1/4 teaspoon unflavored gelatin
1 Tablespoon water
1/2 cup real mayonnaise

In a small saucepan, mix gelatin and water; let stand for 1 minute. Stir over medium heat until gelatin is dissolved. Remove from heat.

Stir in mayonnaise. Spread thinly on crackers, toast rounds, bread, etc. Top as desired to make canapes. Cover and chill up to 3 hours.