Your goal in sealing is to get a thin coat of shellac down into the pores of the wood as a barrier. This is a natural substance, the product of the Indian Lac beetle, and was widely used in colonial days for this purpose. At this point your stock will look like hell. All plastic shiny and probably rough looking. Don't despair, it's going to look worse before it gets better.
All wood has pores.
Walnut and cherry are medium pored, and maple shows small pores. Until these are filled the final finish will appear rough no matter how well the sanding job was done. There are fillers advertised that will quickly fill the wood pores. Unfortunately they also tend to leave a semi opaque material in the pores that blocks the natural beauty of the wood. These are the fillers that advertise that they can be applied and then wiped off leaving behind material that fills the pores.