Finishing is a dangerous subject and an even more dangerous activity depending on your medium and your chosen chemical for the finish itself, and how you think you’ll apply this potential finish, but, there are consideration that must be faced before you apply a finish.
Your subject, or finished painting in this case, is something that you put A LOT of work into to get it where you want it to be, so why, WHY would you risk destroying this painting, without any concern, by putting the wrong substance or chemical on your creation, and why would you pick the wrong technique to apply this substance.
All things being relative, some mediums are more hardy to bad application or incompatibility, at least for a bit of time. You might think you’ve got it good, but as a few months pass, your beautiful painting has gone yellow, or glazed, or even veined with such serious cracking that you think you’re looking through a shattered window. This is not good!
Certain research, plans, testing and so on HAVE to be done. Proper materiels can be purchased but must still be tested, and yes, this is painstaking and not for the impatient of week of heart. Some substances, especially brush on, can be very aggressive, for example, to penwork and pigments such as gouache and liquid acrylics. Brushing a varnish in this case can pull your pigments right off the canvass even if this varnish is compatible. Test for this!
I, personally prefer a brush-on varnish, but in these cases, I will always use a spray on isolation coat as a protective finish so I can brush varnish to my heart’s content. My usual pattern is this – one fine but sufficient sealer coat of spray and two coats high-gloss brush-on. The thicker the finish, the greater risk of problems appearing later…
Here is more information about the all-important “isolation coat. I usually don’t endorse any particular line of product or even stick to using one brand of anything, even if I do have my favorites, but with a finish – the Golden products are the only product to be considered.
Please read this article!