How to Measure & Fit MDF Window Boards (White Primed)

Our primed MDF window boards include a rebated back edge to help bridge old silicone/plaster at the frame. Do not cut the bullnosed front – if depth needs adjustment, trim from the back and re-form the rebate.

• Measure the width (left–right) of the opening at the front edge of the reveal.
• Measure the depth from frame to front of reveal.
• Decide if you want a front overhang (e.g. 10–25 mm) and/or a deeper projection for a stronger reveal line.
• Select the closest stock depth (250 / 255 / 300 mm).

• Cut the width to suit – fine-tooth hand saw or circ saw with guide.
• Never cut the bullnosed front. If reducing depth, trim from the back edge only.
• After trimming depth, re-form the rebate (scribe a shallow notch) so it still bridges any silicone/plaster at the frame.
• Lightly sand all cut edges for a smooth, paint-ready finish.

Seal all cut edges (including the re-formed rebate) with an MDF primer. This prevents moisture ingress and gives an even top-coat.

1. Dry-fit first. The back rebate should sit over any old silicone/plaster line at the frame.
2. Apply a suitable grab adhesive on the underside and to contact points.
3. Press into place, check for level and front projection/overhang, then weight/clamp until set.
4. Run decorators caulk down both reveals for a neat finish.

• Lightly de-nib, then apply your chosen top coat over the factory primer.
• If you’ve oversized the front for a reveal, keep the line consistent across the window.
• Consider adding a small moulding under the nose if you want a more traditional look.

• Store boards flat & dry before fitting.
• Prime cut edges and re-formed rebate with MDF primer.
• Allow a small front overhang if desired (10–25 mm typical).
• Dry-fit and check level before adhesive cures.

• Don’t cut the bullnosed front.
• Don’t leave unsealed MDF edges exposed to moisture.
• Don’t force the back tight to messy silicone – use the rebate to bridge it.