We have very high cathedral ceilings which therefore have very tight angles where the walls meet the ceilings (around 45 degrees) how can we join these? The trowel won’t reach the tight angle
This is plaster not drywall, correct? You do not use drywall techniques on plaster. Drywall the point is to get the mud flat to the board. Plaster is sculpted not drywall mudded. Here's a post from Fine Homebuilding on how to do it:
You get each wall plane flat. We snap a vertical chalk line near the edge on one side and use it as a guide to straighten (flatten) the other side. Once that side is plastered, we snap a line on the other side and plaster to it. Though all purpose mud could never go on thick enough to accomplish this, you could use durabond mixed thickly to make up most of the thickness and later finish with regular joint compound. We often plumb the walls as well. Skim coat plasterers much like drywall tapers do little to straighten anything. Veneer plaster works best going on thin and compound is extremely thin. We often plaster over blueboard with a base coat and then use a lime and gauge finish which can go on heavier than veneer finish plaster. We plaster in three dimensions, vice two. In quality construction, it is expected.
Good luck. If you have never done plaster and have a drywall background you are in for a surprise I think.
PS you can disregard this if you are one of those folks that uses the word "plaster" when you mean "drywall". Although, google up the thread on Fine Homebuilding since most of the discussion is about joining drywall at a 45, not plaster. Most of it is inapplicable to plaster of course.