Best building board by far is what they call a solid core skid door. That’s the door on top of a skid of doors that have marks/damage from the banding holding the skid of doors together. I picked mine up for 25 to 35 bucks each. I also use them as bench tops all around the perimeter of my shop. Nice to be able to bolt a large vice to the bench top and beat on it without worrying about things coming loose. Forget the wimpy hollow core doors just the smooth solid core doors are the best.

For covering the door and creating a building surface that is the best bar none i screw 1/2 inch thick sheet of Homasote to the door. Do not glue or the dissimilar materials will warp just use drywall screws every 10 to 12 inches and this will allow enough movement in the Homosote so the door will not warp. Mine have been in use for over 30 years. When you get a blob of glue you just pluck it off and touch up with your sanding bar. It holds pins beyond anything else yet it’s easy to put them in and take them out. For years many of the model railroad enthusiast have used it as a base to hold their track in place with small pin like spikes. When one side gets so abused you just flip it over and use the other.

Dennis