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How to Join Frame Moulding with an Underpinner
Before joining ensure the frame moulding has been precision cut with 45° mitred edges. The best way to achieve this is by using a Mitre Guillotine, leaving smooth perfect 45° edges.
To accurately join the mitre moulding lengths, use V Nails (also known as wedges) applied with an underpinner. V Nails have a sharp and blunt end, for most one face is silver, the blunt face. The other face is dark, is the sharp face. The sharp end is fired into the 45° mitred frame mouldings. They are manufactured from hardened steel (hard wood) or softer steel (softer wood). A variety of heights are available, the thicker the frame moulding, the taller the selected V Nail should be.
Before inserting V Nails, PVA wood adhesive should be applied to the ends of the mitred moulding lengths. This holds the join together and improved the strength of the V Nail frame join. Take care not to applying an excessive amount of glue as this will be pushed out onto the face of the moulding. It's best to spread the PVA thinly to cover the face of the join.
Select a V Nail based on the thickness of the frame moulding, one that is around 3/4 of the height of the moulding is recommended. If shorter V Nails are used the joint will lack strength and is likely to fail and open. If a longer V Nail is used then there's a risk it will be pushed too far into the moulding and will protrude through the face of the frame, damaging the moulding.
Multiple V Nails can be fired on top of one another, this is useful if a particularly deep rebate moulding is being joined. Take care not to stack the V Nails too high to avoid damaging the moulding.
Repeat the process to join every corner of the frame moulding.