Varnishing your paintings is a very important step, not only does it protect and preserve your painting but it helps enhance the colours. First and foremost however we need to pick the right varnish for our pieces and to do that we need to know what our options are. Two things to keep in mind is the level of sheen you want to see, and how you want the varnish to affect the the colours of your work.
Matte - the colours are softened and have a low sheen.
Gloss - the colours are more vibrant and have a sheen.
Satin - this varnish in an in-between of both gloss and matte varnish.
You can apply varnish in one of two ways, either by spray or by brush. When using the brush method for varnishing you do not require any extra ventilation. With a spray can however you will NEED to varnish in a WELL ventilated space.
When varnishing an acrylic piece you should wait at least one week before varnishing it. This will give the paint plenty of time to fully dry and prevent cloudiness.
Isolation coats
An isolation coat is a layer of acrylic gel or medium that MUST be applied BEFORE you varnish. This coat makes an even surface for the varnish to adhere too as well as protect the painting itself from the chemicals in the varnish that can deteriorate it over time.
Note: isolation coats are PERMANENT you should test the coat on a similar material before using it on your piece.
Let your isolation coat dry for about 24 hours before varnishing.
Varnishing
Now that our piece is prepped and ready we can begin varnishing. Apply a thing coat of varnish to your piece, multiple thin coats are better than one thick coat. After your first coat let it cure for about 2-3 hours then apply your next coat. You should only need about 2-3 coats.
Oil Paintings
When varnishing an oil painting it needs to dry 6-12 months BEFORE varnishing. If you're on a tight schedule, for example your client needs it immediately or you have a before then you can use something called re-touching varnish. You will still need the painting to dry for a couple of weeks before applying this varnish but it's a good compromise in comparison to the 6-12 months.
To start you'll want to lay your oil painting down on a flat surface in a clean and well ventilated room. Make sure to pick a room that you can stay out of for a few hours.
Just before varnishing you'll want to take a clean rag made out of a t-shirt like material to gently brush off any dust that has accumulated over time.
As you start brushing on the varnish you may notice hairs from your brush being left as you go, make sure to remove these hairs IMMEDIATELY as you go. Not after. Varnish dries very quickly.
For large paintings you'll want to work in sections or quadrants. Once your finished your first layer of varnish be sure to leave the room for 2-3 hours to let it dry. If you're in a room with a little bit of dust or animal hair have a box handy to cover your painting to prevent any residue sticking to the painting.
Once the varnish has been applied leave the painting flat so the varnish doesn't run. Finally let your varnish dry fully for 24 hours.
Some Final Notes
If you're looking to varnish sooner rather than later you can apply a thin coat of retouching varnish.
When varnishing make sure to use a clean brush that's never been used for painting. Some people will have a dedicated varnishing brush to avoid any mix ups.