The 69th-ranked Andujar lost his only previous final last year at Bucharest, which was also held on outdoor clay.
Both Spaniards struggled with their serves in a scrappy match that lasted nearly two hours. Andujar broke the 24th-ranked Montanes' serve five times and lost his own three times. Andujar saved 13 other break points.
In the second semifinal, Starace broke Hanescu's serve five times and dropped his only once in the second set.
Hanescu, last year's runner-up, struggled in the decisive tiebreaker and Starace closed out the win on his first match point.