Whenever it's time for me to varnish my acrylic paintings, I get a lot of questions from people about what an isolation coat is.
In short, it's the barrier between the painting and the varnish.
How is it applied?
Golden Paints sell the isolation coat in a handy pre-mixed bottle that you can apply with a brush. It used to be sold as Soft Gel Medium and you had to dilute it 2:1 with water, which was fiddly and a bit annoying. However, the bonus was that you knew it was just a regular medium that was sealing the painting, and nothing fancy or unusual. It's applied in the same way that you would the varnish layer; with a clean soft brush, brushing in all directions.
Why use it?
Golden recommends using an isolation coat so that you don't get 'milky' patches where the varnish (in particular the Matte Polymer Varnish) sucks into parts of the painting unevenly. It also provides a barrier so that if you need to remove the varnish (note: for removable varnishes only...!) you can do so without worrying about affecting the painting layer underneath.
Do you have to use it?
I know some artists who don't, however, Golden strongly recommends it, and frankly, I'm inclined to trust them because they know their sh*t.
Downside
The downside is you have to wait a full 24 hours before being able to varnish over the top of this bad boy. PLUS Golden products are seriously hard to get hold of in Australia at the moment due to covid/global supply issues/war/other.
Have you used an isolation coat? Do you varnish in a different way?
Let me know in the comments.