I recommend the application of an isolation coat before applying an archival varnish. In oil painting an isolation coat may or may not be needed, but in acrylic painting it is essential.
This article was first published by me in 2016 on evilgenius.space (now defunct). It differs from source material provided by manufacturers in so far as they rarely mention unwelcome characteristics of their other products. Based on the type of advice you may encounter, one can tell whether other websites have just regurgitated commercial information or perhaps were 'informed' by web sites like this one. On this site, I write as a practicing painter and organize the information from the point of view of my own problem solving.
Bathed in Sunlight 24X36"
Acrylic Painting (cropped)
BEFORE STARTING - CLEAN IT
Do not use a dust encrusted vacuum nozzle if your canvas has any texture whatsoever. You will add dust and hair to your painting. A needle imbedded in a wood handle or a cutting blade can come in handy. A lint roller or masking tape works. A microfibre cloth has less lint. As well, a water spray bottle if you have to wipe it off before reapplying one of the coats. If your painting and canvas are really well constructed, you could just shower the surface in the bath tub, but if you use regular painting methods or common frame construction, I advise you not to.
All but the last coat should be clear (gloss). The final coat is archival varnish and can be clear as well if you wish. However, if it is either satin or matte, it should be applied in a single, even, final coat so that it does not obscure viewing of the painting.
FIRST COAT for this painting consisted of Grumbacher Permanent Gloss Varnish thinned by one third distilled water and applied with a new sponge brush as the canvas lay in a horizontal position. Ensure no foam or bubbles. Inspect for complete coverage and then air dry. This is not an archival varnish because it is permanent. This coat serves to fix the paint washes to the canvas and stiffen the canvas weave in much the same way as a priming coat does on a raw canvas or even an underprimed canvas.
Sponge-backed sandpaper (3M Super Fine - made in England) was used to lightly sand the rough surface before applying the next coat. This is so your sponge brush will not be shredded in later applications. I applied the second coat of the same permanent varnish to the dry-looking background areas only. These areas need more varnish because they are more absorbant.
Together the first two coats prevent subsequent coats of gel from soaking in. In the case of a final matte varnish which contains a matte additive, uneven distribution of that matte additive may otherwise collect on the surface and this will be avoided.
THIRD COAT is a foam brush application containing about 2/3 Golden Self Levelling Gloss Gel and one third thinned varnish mixture from above. Golden Soft Gel has been recommended elsewhere and could be used in place of the levelling gel that I have used
FOURTH COAT consists of the same gel mix but was applied with a wide #50 Simply Simmons synthetic brush to make the coat thicker. The purpose of these 2 coats of acrylic gel is to:
1. Form the isolation layer separating the removable varnish from the painting surface.
2. The objective is to even out the glossiness of the canvas surface before applying the final varnish. You should use a thinned gloss gel because it is clear and the evenness of a gloss surface will ensure that any non-gloss varnish applied later will also be absolutely even.
FIFTH AND FINAL APPLICATION
This coat is the removable or archival varnish, Golden Polymer Varnish with UVLS (Satin). It may be slightly thinned with distilled water but I found it unnecessary. I shook it to disperse the matte additive and banged it down on a hard surface a few times to get rid of the bubbles. My foam brush applied it so effectively that only one coat was needed. The reduction of glossiness provided by one coat was sufficient. The intent of the method used here is to keep the effect of the overall coatings relatively clear by using only a single satin application as the final layer to reduce the glare on the surface coat.