Exterior decorating advice needed please. I've finally got round to starting to paint the shed door. It's a 4' x 6' bit of ply on frame of 4" x 2" rough-sawn softwood. The outside looked like it had been under-coated previously so that got sanded down and another coat. The frame and back of it are raw timber still. I'm only bothered in how the outside looks but think it'll warp or try to if only one side is painted. Do I need to put primer on the insides or will a couple of coats of undercoat suffice? The outside is getting another undercoat and then some paint and varnish. Thanks.