When
Dutchman-ing, I've found using 3" masking tape will work but you want the blue stuff. I'm also a big fan of using Latex Painters Caulk. If you build a set in a shop, slice the seems, load it onto a truck, then re-assemble in the
theatre , the caulk can be a God send. Internal seams on a flat are best dealt with, with Bondo, as Joint compound tends to be too fragile for transport.
But if everyone is married to the tape Idea, the wider the better.
I don't think Dutchman is in the Wikki yet I'll have to fix that. Dutchman-ing flats used to be done with strips of
muslin dipped into glue. You would apply the dutchman over the seam in question, working out all the bubbles with a paint brush. you would then paint over everything. Worked really well in bigger theatres. Now days we tend to spend a whole lot more time working on seams, as the set I'm working on right now would attest to.