Nail long panel pins through each corner to strengthen the joints. To prevent the wood from splitting, use the sharp point of a compass to make a pilot hole for the pins. Use a fine gauge nail punch to push the heads just below the surface of the wood – or use a masonry nail with its point filed down.
Once the glue has set, release the frame from the clamps. Add any decorative finish at this stage, while all surfaces are still accessible.
Ask a glazier to cut a sheet of picture glass to size. Position the glass inside the frame and tape the gap between the glass and frame. Take care that the tape does not encroach on the glass where it could be visible from the front side. The tape will prevent dirt, dust and tiny insects from entering. Self-adhesive tape, such as masking tape, is not suitable for framing or conservation jobs because it will dry out and fall off – use only plain brown paper tape that sticks when it is wetted.
Draw up the width of mount to surround the picture area on the reverse side of a piece of acid-free, 6-ply mounting board. Draw a line exactly 25 mm (1 in) inside the first line and allow the corners to intersect. Use a precision board cutter to cut the mount: insert the cutter in its groove on the precision ruler and place the measuring edge along the innermost line. Run the cutter between points A and B (see left), which are aligned with arrows A and B on the cutter.
Use the mount as a template to cut a backing card for the artwork (use a minimum 2-ply card; it must be acid-free, otherwise it will damage watercolours and prints). Join the mount and backing card along one edge with linen tape and an animal glue adhesive, or use paper tape.