On many older types of furniture,especially pieces that were made of,what we here in the UK,call, yellow pine or Victorian pine...or to give it its proper name Oregon Pine,..Gesso was used to prime the surface,.and it covered any imperfections in the wood,such as knots and irregularities in the grain.this is a primer using a binder,e.g. liquid glue,chalk,gypsum,the wood was painted then a varnish was applied,...then a regular grain could be made by the use of a combing effect....a bit like the application of Paint and Grain,that was popular in the 1980,s I am at present restoring an old Pine Kist....or Blanket Box as it is sometimes known,but here in Scotland it is called a Kist,...when I got it it was varnished,to make it look like old dark Oak,..this was evident when even the old steel hinges had varnish on them with a grain running through,..the surface of the Kist,looked to be perfect, but when i stripped the varnish off,i uncovered the yellow Gesso primer,and when this was removed, i could then see Knots in the surface,that were not evident when i first got the Kist,...I will post pics in a few days to show what I mean...