How to glue uPVC trim to an MDF window board

uPVC is a low energy plastic with a smooth extruded skin, often carrying a mould release or a protective film residue. Most people skip cleaning it and that is why trims come loose. A wipe with isopropyl alcohol and a light scuff with a scouring pad takes seconds and changes the outcome entirely.

MS polymer is a good fit for this joint for three reasons. It has no solvent in it, so it will not attack the uPVC surface or the paint on the window board. It cures to a slightly rubbery solid, so the small seasonal movement of the MDF does not tear the bond line. And once it has skinned it takes paint, which matters on a trim that will be decorated.

MDF is only difficult where it has been cut. A factory face takes adhesive well; a raw cut edge is absorbent and should be sealed first. Because MS polymer is thick and does not soak in the way a water-based glue does, the sealing step matters less here than it does with PVA, but it still helps on a fresh cut.

1. Dry fit the trim and mark its position with low tack tape.
2. Wipe the bonding face of the uPVC with isopropyl alcohol, then scuff it lightly with a grey scouring pad and wipe again.
3. If the MDF edge is freshly cut, seal it with MDF sealer or diluted PVA and let it dry.
4. Dust off the window board where the trim will sit.
5. Run MS polymer along the trim in a bead about 5mm wide, or two beads if the trim is wide, keeping back from the edges.
6. Press the trim into place, pull it off, wait a minute, then press it back and slide it slightly to bed it.
7. Tape it every 150mm and wipe any squeeze out with a dry cloth before it skins.
8. Leave the tape on for 24 hours before painting or loading.