I finally managed to complete my quatrefoil stenciling project! Just a warning - this isn't one for the fainthearted. It wasn't difficult but time-consuming, especially as painting the back of a bookshelf is very fiddly. Still, I found it oddly relaxing and my stencil is still intact so I'll be reusing it again very soon I'm sure.
I first created the above stencil in Photoshop (with a lot of help from DH). I've done a couple of other sizes and you can download all the templates here.
I then printed the stencil out on a sheet of A4 card. With a marker I drew in the connecting bars that you can see in the above image (without which the middle of the design would obviously fall out) and cut the remainder out with an Xacto knife. A boxcutter also works reasonably well but doesn't cut as cleanly.
I then primed the back of the bookshelf with White Knight Grip Lock Primer, which allows paint to adhere to laminated surfaces, and painted two coats of brown base colour using a mini-roller for a smooth finish.
Using a thin paint brush, I stenciled the design in white paint over the brown base using masking tape to hold the stencil in place. I was a bit gung-ho initially and tried the mini-roller but the excess paint bled through the edges of the stencil and looked very messy so I painted over my first attempt with the base colour and tried again with the brush. Make sure you don't have too much paint on the brush either or you risk the paint bleeding under the stencil.
The A4 page fit roughly three and a half times in each shelf. Where I couldn't use the stencil because the corner was too tight or awkward, I simply hand-painted the design doing my best to preserve the symmetry. It isn't perfect but the areas where I had to do this are around the edges of each shelf and not really very visible. If you were painting a flat surface this wouldn't be a problem.
I did a final touch up using the brown base colour in any areas where there was a bleed. In the gaps where the connecting bars were in the stencil I just applied a little circular dab of white paint to make the design look connected and cohesive. I am SO pleased with the final result...
So my parents' 30 year old bookshelf went from this:
to very chic bathroom storage...
and now hopefully has another 30 years in it!