If you install thin wood paneling backwards over drywall and then paint the back side of the paneling, the result is likely to look quite different from traditional drywall. Here are some potential outcomes:
Texture and Appearance: The back side of thin wood paneling may not have the smooth and uniform surface that drywall does. It might have visible wood grain, knots, or other imperfections that would be unusual for a typical wall surface.
Seams and Joints: The seams between the wood panels may be more visible than typical drywall joints, depending on how well they are installed and aligned. Drywall seams are usually taped and mudded to create a seamless appearance, but this may not be as effective with wood paneling.
Color and Finish: If the wood paneling is painted on the back side, the paint may not adhere as well as it would to a smoother surface like drywall. This could result in an uneven or streaky finish.
Durability: Thin wood paneling is not as durable as drywall when it comes to wear and tear. It can be more prone to damage from impacts, moisture, and other factors that may affect its appearance over time.
Aesthetics: Whether it looks "awful" or not is subjective and depends on personal taste. Some people might like the rustic or textured appearance that wood paneling can offer, while others may prefer the clean and smooth look of traditional drywall.
If your goal is to achieve a smooth, uniform, and traditional wall appearance, using thin wood paneling backward is not the best choice. In that case, it's better to either install the wood paneling correctly (with the finished side facing out) or remove it and replace it with drywall for a more conventional look.