I am traveling to visit Findlater Castle in Cullen in late May 2019 from Aberdeen. I would love to hear advice on the best method of traveling (except driving).
I think you can take a bus to Portsoy, but after that, it's going to be a taxi, Uber or shanks' mare. I would advise contacting the Tourist Information center for Aberdeen and see if they have any suggestions. Their email is Aberdeen@visitscotland.com
The number 35 Stagecoach bus from Aberdeen bus station (Union Square), near the harbor, goes through Cullen several times a day during the week, and less frequently on Saturday and Sunday. The journey time is just over 2 1/2 hours, but you'll see some wonderful scenery along the way. Findlater Castle is a bit east of the town; perhaps you could ask the bus driver to let you off as close to the castle as possible. If there is a bus stop for Sandend, possibly. You'd probably have to walk in to Cullen to catch the return bus.
p.s.: WalkHighlands has a route description from Cullen. You can access it from the WalkHighlands website.
p.p.s.: Undiscovered Scotland website has a reference to Findlater Castle on their website. The comments are interesting.
p.p.p.s.: If you have time on the way back to Aberdeen while you're waiting for the bus, try some cullen skink at the Cullen Bay Hotel.