Is it necessary to paint all sides of interior trim before you install it? Even the back side that you don't see?
I'm installing a 1x6 Pine ledge over a knee wall. It's a 9ft length of pine, and I'd rather not see it warp 6 months after installing it.
Will painting all sides make interior trim more stable or am I just wasting paint?
Painting the whole thing would keep the moisture content of the wood more stable, so it would be less likely to warp. However, don't expect miracles from pine. Be sure to prime it well if it has knots or those will bleed thru just about any paint. They make hardieplank in 1x6
It wouldn't hurt to prime all six sides, but it won't hurt if you don't. Exterior, absolutely prime as much surface as you get access to. BTW, prime the knots, if any, with BIN pigmented shellac. In fact, you can prime the whole board with BIN. Outside, no, BIN is only a spot primer.