Hi Judy—Please never apologize for any number of questions. That’s what all of this is here for.
Regarding Liquin as a final coat over the medium that you use: I cannot speak to that exact scenario with any experience. However, I can give you some food for thought that may help you to navigate the issue for yourself. I should state that while I use Liquin for very specific reasons—I try not to promote it’s use in this fashion (as a final coat) as many conservators and material gurus rail against it. If you would like to read my full reasons for using the material in this way I have a comprehensive article here:
Additionally, a common heuristic involving mediums and varnishes is that you should never varnish with the same material that was used as a medium. The reason being that future restoration that involves the removal of the varnish may adversely impact the integrity of the paint film (containing the same materials.) However, some rule-of-thumb guidelines are being challenges by some contemporary experts. Here is a recent post from Natural Pigments’ George O’Hanlon regarding some commonly held (and sometimes confusing) concepts—including one that may be relevant to your question.
Confusing Concepts in Oil Painting: Fat over Lean… Thick Over Thin… Thick Over Lean… Whatever…
It’s Time to Get Rid of Them. - By George O’Hanlon
Let’s toss out the concepts of “fat over lean” and for that matter “thick over thin” (or the confused “thick over lean”) while we are at it, and let’s consider the physical structure of the paint.
To help you to understand the properties of oil paint, it is helpful to understand the relation between the pigment and oil. One way to think about the relationship between pigment and binder is a brick wall. Every mason knows there is an ideal ratio of mortar to brick. Too much mortar and the wall is weak. Not enough mortar and the bricks fall apart.
The same relationship exists between the pigments and binder in dried paint. We call this relationship or ratio the pigment volume concentration or PVC. Pigment volume concentration (PVC) is the volume of pigment compared to the volume of all solids. If paint has a PVC of 30, than 30% of the total binder/pigment is pigment and 70% is binder solids.
The point at which there is just enough binder to wet pigment particles is called the critical pigment volume concentration (cPVC). For almost all colors this is between 45% and 55% PVC. Films with lower concentrations of pigment have more gloss but as the PVC increases they become increasingly matte. Films with high percentages of pigment are more permeable to moisture and susceptible to solvents. This is because with more pigment, there is less binder to fill the voids between pigment particles. This porosity leaves the film open to the environment. Films with higher pigmentation have increasingly lower tensile strength. (See the graph.)
As the PVC increases past the cPVC, the dried paint film will contain voids. The porosity will increase, moisture vapor transmission will increase, wet hiding will decrease, but dry hiding will increase. As the paint dries, a condition will exist where the paint has too much pigment and too little oil such that the internal voids created will entrap air or solvent in the vapor phase.
Most paint in tubes contain enough binder to wet and envelope pigment particles and so are at about the critical pigment volume concentration or cPVC for that pigment or mixture of pigments. The variation in the pigment volume of particular colors between brands are due to the type of oil used, the amount of oil (some paint makers may choose to make their colors softer or thicker) and the amount of additives used in their formulation.
So the best way to discuss paint is by understanding this relation between pigment and oil which is expressed as the pigment volume concentration (PVC). Oil paint straight out of the tube is in the form of a paste and is usually near its cPVC. Adding oil to paste paint lowers the PVC (low PVC), while adding solvent potentially increases the PVC (high PVC). Hence the ideal paint is a paste paint and this is why the admonition to “apply thick paint thinly” or work with paste paint in thin layers, is the best practice for oil painting.
From this it is clear that paint consists of both liquid and solid components. In the case of oil paint, the liquid component is a drying vegetable oil. The oil undergoes chemical and physical processes that change it from a liquid to a solid, which process is called “oxidative polymerization”. Oil is the binder or glue that holds the pigment particles together and on to a substrate.
You may think of paint as pigmented or colored adhesive. When it comes to adhesion in oil paint, oil is the glue. While many artists believe that mechanical adhesion is most important when it comes to paint, what is more important is “dispersive adhesion”, which does not rely on absorption of the binder or oil into a porous substrate substrate or on surface texture (although they can improve dispersive adhesion) but rather on surface energy and polarity. Hence, paste paint adheres better than paint that has been heavily diluted with solvent (high PVC) even if it is partially absorbed into the substrate, because the latter has less oil or glue to adhere pigment particles to the surface of the substrate. -George O’Hanlon
