Anything that you put on the inside of your shell has the potential to change the sound of the drum. That could be a good thing or not so much. Do you want to change the sound? If you use a hard sealer, like a lacquer, varnish, or polyurethane, you will make the drum brighter by creating what amounts to another hard ply on the inside of the shell. As an extreme example, look at the Pearl
wood/fiberglass shells of the 70's. They used a luan/Philippine mahogany shell, which is a fairly soft wood, and then spun a layer of fiberglass into the inside of the shell. The hardness of the FG created a brighter drum that projected much more than the luan shells were capable of. Many other companies have used sealers inside their shells, ranging from clear lacquers to paints. Look at the white-painted interiors of the 60s-70's Ludwigs
, grey speckled Rogers, and the famous silver paint inside those great Gretsch
shells. If your goal is simply to protect the wood, try rubbing the inside of the shells with a furniture-grade lemon oil. I did this with my 1982 DWs
, to no detriment.