PCS union members at Amgueddfa Cymru/National Museum Wales are set to begin a series of strikes next week.
Workers at seven sites are in a dispute over proposals to cut the pay of the poorest-paid staff.
Bosses want to scrap payments for weekend working, which make up as much as 15 percent of front-of-house staff pay. Some workers could lose up to £3,000 a year.
Payments would remain for bank holidays but would no longer be pensionable.
Bosses have threatened to dismiss staff who refused to sign up to the changes, then re-engage them on the inferior terms.
12 & 19 August, Slate Museum and Cathays Park
13 & 20 August, Drefach, Swansea and St Fagans
15 August, National Waterfront Museum
15-16, 22-23 & 30-31 Aug, Big Pit Mining Village
29 August, all sites
PCS members will also take part in action short of strike from 12-31 August