A homeowner has a brand new uPVC window on a recent extension. A scaffolder dropped a tube which put a deep scratch across the glass and left a gouge in the frame – roughly 10mm long, 5mm high and 1-2mm deep. The pane will be swapped out, but they'd rather not remove and refit the whole frame just for a small mark. The question is whether the gouge can be repaired in situ, and if so, what product and method actually holds up.
This is a common one on new builds and extensions. Before you start filling and sanding, it's worth understanding what's happened to the plastic, because that dictates how you tackle it. I'll run through what's going on, the checks to do first, and the good/better/best options, plus the bodges that look fine for a fortnight and then let you down.
What's happened to the frame
uPVC is a solid plastic profile, not a painted skin over something else. When something heavy hits it, two things happen at once. Some material gets torn out and lost, and some gets pushed sideways and sits proud around the edge of the gouge. That raised lip matters. If you start sanding or filling straight over the top, you'll be working with a false surface and the repair will always look like a lump.
The other thing to know is that the white you see is the colour of the plastic all the way through, on standard white frames at least. That's good news for shallow marks, because sanding doesn't expose a different colour underneath. On foiled or coloured frames (oak effect, grey, anthracite) it's a different story, because the colour is only on the surface film. Sand through that and you're into white, and no amount of polishing brings it back.
Simple checks before you commit
Do these first so you know what you're dealing with:
- Run a fingernail across the damage. If it catches on raised edges around the gouge, that's displaced material sitting proud, and it needs paring back before anything else.
- Look at the frame in a raking light from the side. This shows up the true profile and any high spots you'd miss looking at it straight on.
- Check whether the frame is solid white uPVC or a foil-wrapped/coloured finish. Scratch a hidden spot gently with a blade. If white shows under a colour, it's foiled and you should not sand it.
- Confirm the depth with the corner of a steel rule laid flat across the damage. Anything under about 1mm is usually sandable; deeper than that and you're into filling territory.
- Check whether the damaged section is a fixed frame or an opening sash. If it's a sash, a replacement can often be ordered from the original installer far more cheaply than swapping the whole window.
Why most DIY repairs go wrong
Most DIY uPVC repairs fail for the same reason: people fill over displaced material, sand the filler flush, and end up with a shiny patch surrounded by a ghost outline of the original damage. Or they sand hard without working through the grits, leaving fine scratches that then hold dirt and go grey within a few months. Water and grime get into any scratch you leave behind, and once it's in there, wiping the frame down won't shift it. That's why the prep and the grit progression matter more than the product you finish with.
Proper fix – good, better, best
Good: sand and polish only
If the gouge is shallow and mostly displaced rather than missing material, you can often get away with no filler at all. Pare off the raised edges first with a sharp chisel or a new Stanley blade held dead flat to the surface. No sawing action, just careful slicing. Then work through wet and dry paper with plenty of soapy water: start around 400 grit if the damage is light, or 120-240 if it's heavier, and work up through 400, 600, 800 and finish at 1200. Feather out well beyond the damaged area so you don't leave a dish. Finish with a mild car polish or a light cutting compound. On solid white uPVC this can be near invisible.
Better: fill, sand and polish
Where material is genuinely missing, you need a proper uPVC repair filler. The trade ones used for cosmetic window repairs are two-part products designed to bond to PVC and sand cleanly. Don't use car body filler or general-purpose polyester filler; the bond and the colour match will let you down. Pare back any proud edges first, clean the area with a PVC cleaner or isopropyl alcohol so nothing contaminates the bond, then apply the filler slightly proud. Let it cure fully. Follow the data sheet, don't rush it. Then sand back through the grits as above. Because most fillers aren't a perfect white match, this method usually needs a light touch of matched spray paint over the repair to blend it in. That's where patience comes in.
Best: replace the affected section
If it's an opening sash, order a new one from the original supplier and swap it over. Straightforward job, perfect finish, no compromise. If it's the outer frame and the damage is in a prominent spot, a professional uPVC cosmetic repair specialist (they do exist, often marketed for new-build snagging and end-of-tenancy work) will do a filled and sprayed repair that's genuinely hard to spot. Given the scaffolding firm is paying, it's worth getting a quote from one before you pick up a bit of wet and dry yourself.
Bodges and mistakes to avoid
- Filling over raised, displaced plastic. You'll always see the outline.
- Using car body filler or decorators' caulk. Wrong bond, wrong colour, wrong flexibility, it'll crack or yellow.
- Skipping grits (jumping from 120 straight to 1200). The coarse scratches stay in and hold dirt.
- Dry sanding. It loads the paper, burns the plastic and leaves swirl marks.
- Sanding a foiled or coloured frame. You'll go through the colour layer and can't put it back.
- White silicone smeared into the gouge. Looks alright for a week, goes grey, peels, and makes a proper repair much harder later.
When to bring in a specialist
If the frame is foiled or a non-standard colour, don't attempt it yourself. Get a cosmetic repair specialist in. Same goes if the damage is on a highly visible elevation and you want it truly invisible. And if you spot any cracking that runs into the frame rather than just a surface gouge, that's a structural concern with the profile and the window may need replacing regardless of how the cosmetic side looks.
Reality check
A good sand-and-polish repair on solid white uPVC takes an hour or two and costs almost nothing. A filled and sprayed repair is a half-day job by the time you've cleaned, filled, cured, sanded, masked, sprayed and let it flash off, and it needs a dry, still day if you're spraying outside. Curing times on the filler are not optional; sand too early and it'll drag and smear. If the scaffolder's insurance is footing the bill, get two quotes, one for a specialist repair and one for a replacement sash or frame, and let them decide. Nine times out of ten they'll go for the repair, and you keep the new fixings and seals undisturbed, which is what you want on a brand new extension.