Almost any finish will change the of plain raw wood when finish is applied. Generally oil based finishes will change the look more than water based finishes. Oil based finishes will darken such as turn grey to black; it will also amber by turning wood tones brighter and colors will pop. Straight water based poly acrylic such as General Finishes tends to change the wood the least. Several coats of this will help encapsulate slivers if buffing between coats and make it feel smooth but look textured. Also use as dull of a sheen as possible to avoid the glossy look. Always test a sample. Our warranty on our products is void if the product is not finished and maintained. We would never recommend not finishing any exterior door especially. If the customer doesn't want the look of finish on an interior door, though, that would be more acceptable. They just need to understand it could be more likely to warp and feel rough to the touch. Paneling is also less in need of a finish. We still recommend finishes, though, even for that to seal and protect the wood. One thing to consider is that one doesn't know the complete previous history of reclaimed lumber. If it was in a barn it was certainly exposed to lots of dust and animals. Finishing it seals in some of the "unknown" particulates still present on the wood.