A spit coat can help you apply an even finish by sealing the wood's grain.
Are you having a problem with some woods, like birch, staining unevenly? This is caused by grain patterns that, when stained, act like end grain and suck up the stain. A mixture of 2 parts Denatured Alcohol and 1 Part Orange Shellac might help. Paint on the mixture and allow to dry, then finish as usual. The shellac helps even out the stain absorption.
Test the spit coat on a piece of scrap to make sure it will work for you.