Sometimes, if a portrait has many layers of pastel or I have used extra soft powdery brands, it may need to be sprayed with a fixative to help hold the pigments in place during shipping. I do this outdoors as the fumes are toxic, and then I ensure the portrait is dry before wrapping. I will often use a hair blower set on "medium" to hasten the process.
Once dry, the portrait is fixed to a rigid lightweight slightly over-sized board (usually foam core) using tape on the corners. I use a metal ruler and a box knife cutter and cut the boards on a self healing cutting mat.
I then protect the surface of the painting using a sheet of glassine paper. You can substitute wax paper if you are really stuck. Make sure the glassine or wax paper covers all of the artwork and is large enough to fold on the back and is secured with tape to the back of the board.
Before waterproofing the portrait (oh yes, trust me on this one!), I write "back" with a black marker on the back of the foam core support, and "front" on a second piece of foam core (or board). I then place the "front" foam core on top of the portrait and "seal" the boards using shrink wrap tape. I start at one end and wrap all around to the other end, then rotate the board 90 degrees and repeat.
Now that my portrait is sealed with shrink wrap, I sandwich the portrait in between another layer of slightly over-sized cardboard, making sure to match "front" to "front" and "back" to "back". If the artwork is pre-mounted to board, I feel usually safe enough to simply seal the edges at this stage and affix the shipping label.
Since the peacock painting I am packing for shipping is done on velour paper rather than board, I add an extra level of protection. I was lucky to have empty Bristol board boxes which act as perfect sleeves to slip my protected painting. I simply slide it in, affix the prepaid shipping label and scribble some warnings on the box with a red marker. NOTE: I ALWAYS insure any artwork that I ship, no exceptions!
The last step involves sealing the box. I first use regular clear packing tape on all sides/seams as it is inexpensive. For added durability, I then tape all four sides with red "Tuck" tape. Voila! Now it's time to head to the Post Office!
So far, each and every client has received their precious cargo intact. In fact, I have received many comments from clients about how impressed and well protected my portraits were packaged! Some even went further, saying they had a hard time unwrapping it! lol My motto: Better be safe than sorry!
P.S. I once had a client tell me their precious portrait was left outside by the garage door during a torrential downpour. I was relieved to learn that although the outer layers of cardboard were drenched, the inner sealed boards as well as the artwork were dry as bone. Phew!