Unless you've got a really long run I wouldn't got to all the effort of caulking and spakling. Masking tape can usually be hidden rather easily by painting the wall with one of the rag effects or bold stipes.
Purchase a primer and an all in one paint/primer (Benjamin Moore's Aura in a matte finish is pricey but good stuff). for the base color for the rag effect. Keep lighting soft and you it should be rather hard to see the seam.
You said this was for Daddy Warbucks If possible buy some outdoor foam insulation and some molding. and make a couple pillars along the seams.
Purchase a primer and an all in one paint/primer (Benjamin Moore's Aura in a matte finish is pricey but good stuff). for the base color for the rag effect. Keep lighting soft and you it should be rather hard to see the seam.
You said this was for Daddy Warbucks If possible buy some outdoor foam insulation and some molding. and make a couple pillars along the seams.