I use LUKAS Oil Painting Mediums - Varnishes which I have bought from JerrysArtarama's Store a year ago. It is a best varnish for Oil painting. It gives a great sheen and gloss to your painting.

My name is Ann Young I use PS for photo restoration works for more than 7 years. I'm a Pro digital artist. I work in professional editing company FixThePhoto since 2013. Earlier I was a freelance photo editor. I like to do everything which is related to photo manipulations, but I prefer portrait editing the most. If you want some more information don't hesitate to write me. 

Thank you for posting this!  This is so very helpful.

Carolyn Brunsdon said:

See if this helps on both the question of Retouch Varnish and the oiling out. (This has been helpful for me as well...too long between serious painting sessions, and even then moved on to the next one instead of preparing the finished one properly.

 

Oiling Out to Even the Gloss or Shine on a Painting
"I have a water-mixable oil portrait that won't be ready to varnish for some months yet, but some areas are glossy from glazes and some areas are flat from lack of glazes. Should I try to bring the entire work into the same gloss range by glazing the other areas now?

Or, if I decide to go with a matte varnish rather than a gloss, will it 'kill' the high gloss areas and give the painting the same gloss all around? Alternatively would a gloss varnish give the entire painting the same gloss?" -- Chandler
Answer:
Painting and Technical Advisor for Winsor & Newton, Paul Robinson, says: "What I would advise is that you even out your sheen before varnishing by 'oiling out'. Oiling out is the application of an oil medium to a painting which has sunk (become dull), or lost its oil to the layer underneath. The most common causes for this are an over-absorbent, cheap ground or the use of too much solvent or water and insufficient or no medium.

"When the colour is dry, oil painting medium should be sparingly rubbed into any sunken areas with a clean cloth. As you have used a water mixable oil, I would recommend Artisan Water Mixable Painting Medium which is a water mixable version of the traditional medium. Wipe off any residue and leave to dry for a day or two. If smaller, dull areas remain, repeat the process until the painting has regained an even sheen.

"In general varnishes should not be used for the purpose of recovering the lustre of a dead painting. A decent varnish would still show the difference in sheen from one area to the next."1
If you oil out with just an oil, rather than oil/solvent mixture, it may take a while to dry. Be sparing with it, not to puddle it on your painting.
Gamblin recommends oiling out using "a liberal coating of 1:1 Galkyd Painting Medium and Gamsol Odorless Mineral Spirits to a dry painting"2 to either the whole painting or just an area, leaving this to be absorbed for about two minutes, and then wiping off the excess.
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And on Retouch Varnish:

Definition:
Retouch varnish is a temporary varnish you can apply to an oil painting when it is touch dry rather than fully dry. Unlike 'normal' or 'final' varnish, retouch varnish allows an oil painting to continue to dry. When the painting is eventually dry, you can apply a final varnish over the retouch varnish (be sure to brush any dust off first!).

Retouch varnish evens out the gloss of a painting, and protects the painting if you want to exhibit or ship it.

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*My niece is an accomplished artist and she ships her painting after using the Retouch Varnish along with an instructional note as to how to apply the final varnish. Otherwise they have to wait that 6 mo. to a year. (If she has a contact in the area where her client is, she has been known to make arrangements with that fellow artist to complete the varnishing and she reciprocates later down the road.)