I'd oil the beams first. Sometimes, water/wet plaster can stain untreated wood/timber, especially mahogany, oak etc. Oil the beams, couple of coats, then use some Frog Tape along the junction where the beam meets the ceiling. Do as much as you can to keep the beams clean.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local