what finish to use

We need a bit more info to tackle this question….first, when you gently drag your finger across the surface (I assume this is on a canvas with an acrylic ground?) do you pull up a bit of color? That will tell you if the paint is truly underbound…

If your paint is underbound then you would certainly want to add a layer of medium on top…preferably not too thick. Consider spraying (rather than pouring) the medium if pigment is coming up on your fingers rather easily when the surface is touched/rubbed. You can continue to test whether or not the paint layer is still underbound after a couple of spray applications. Varnishing an underbound surface can lead to problems in the future, particularly if your painting requires cleaning or if the surface experiences damage (e.g. blooming/blanching of the varnish, discoloration/degradation of the varnish). It is best to put on a layer of varnish when you are finished with the painting process and your paint layer is sufficiently bound.

It looks like you have gotten some good advice below, so let me just really add and amplify to what has already been said. The first thing to realize is that many acrylics – and here I can only draw from the testing we have done with our own GOLDEN Fluids and heavy Bodies – can be thinned up to 1:1 with water and still obtain a good, durable film with excellent adhesion. In fact, when testing the adhesion of our Fluid Acrylics onto Plexiglas, we still had good adhesion at ratios as high as 1:2 and 1:3 with water – although we will admit those are pushing the boundary. The main thing is that you might be surprised that the adhesion you have is fine and the way to test would be to dampen a cotton swab with water and gently rub an area and see if you get any color lift. If so, then locking things down with an isolation coat would be recommended. If you have the ability to spray-apply acrylics, then you can follow the spraying instructions we give on the following tech sheet about Isolation Coats:

Isoaltion Coats

Otherwise, you can apply a couple of very light layers of our Archival Varnish Gloss as a way to lock things down and lessen water sensitivity before brush applying an isolation coat made from two parts Soft Gel Gloss and 1 part water. The use of an Isolation Coat is also recommended as a step prior to a final varnish, so learning about its use can be useful elsewhere as well.

If needing to thin paints down further in the future, and wanting to preserve a strong film, you can certainly use something like ourAirbrush Transparent Extenderor even better, look at something like ourHigh Flow Acrylics which have as much pigment as our other acrylic lines but have simply been formulated with a binder without the use of the usual thickening agents. And while all of these are GOLDEN products – simply because those are the ones I am most knowledgeable about – many brands make similar things that could be worth exploring.

lastly, in terms of pouring acrylics and making cells, you might be interested in this recent article we wrote on the topic in our technical online newsletter, Just Paint: Understanding the Techniques of Pouring Acrylics Towards the end it has a section on using isopropyl alchohol as a way to generate cells that might be of interest and even provides a starting recipe.

Hope that helps and if you have any other questions just ask.

Sarah Sands
Senior Technical Specialist
Golden Artist Colors