Protecting the interior of a dulcimer

Hello all I was just wondering if it is common for builders to apply any kind of protectant to the interior of their instrument before gluing it together? Up until this point it hadnt occurred to me. I have noticed all of my guitars, mando, ukulele, etc have exposed wood on the inside and it occurred to me that this might affect the longevity of the instrument since the interior will be exposed to more humidity than the protected exterior. Would coating the inside with shellac, poly, nitro, or something similar affect the tone significantly? Is there a reason all of the commercially produced instruments I own only have coating on the outside? Thanks for any information, stay safe!