The Couch

Couch - Usually Linseed oil is used as a couch. However I have also used Liquin Fine Detail before hand and then rubbed it off entirely before applying the normal Linseed Couch. I feel this helps in accelerating drying time as well as add elasticity to the paint film. Liquin Fine Detail can also be used on it's own or actually mixed with the Linseed oil though this may become too tacky to work with.

How it's applied (just before painting)

- Rub the medium all over the area to be painted (in later stages some parts may not adhere because of the extra smooth surface (sort of pearling up as water does on an oily surface)... just rub the Linseed/ Liquin in with your finger till it adheres completely). 

- wipe off access with a soft cloth (lint free) till only a sheen of medium is visible over the surface....

Why Couch? applying a couch gives you the opportunity to rub down areas that are too slick, getting the medium (usually linseed oil) to really adhere completely to the area before painting. This ensures that the paint you apply after will also adhere well and completely to the whole surface... A couch often times gives a more desirable surface to work on too...

note* there is also the old masters technique of carefully wiping half an onion or garlic over the surface to help the next layer of paint adhere as well... Though I have not tried this yet I might in the future, and post more details...