The smell hit me before I saw the problem—like old pennies and wet cardboard. I’d cracked open a kitchen cabinet in a 1930s terraced house to swap a basket strainer and found a golf-ball-sized chunk of grey putty sagging off the flange like melted wax. The homeowner, a keen baker, had smeared the stuff around the rim three years earlier after watching a five-minute video that promised “leak-free for life.” She’d never heard the word weep used for plumbing, yet that’s exactly the slow-motion failure her DIY “waterproof” seal was staging. One squeeze and the putty oozed water; the cabinet floor was swollen like a sponge. She asked the same question I hear every month: “Should I have used silicone instead?” My answer never fits in a single sentence, because the real choice is about chemistry, joint movement, and how long you want the repair to outlive your next decorating whim.
I’ve been a City & Guilds Level 3 plumber for 12 years, logged 2,400 sink installs, and keep a photo album on my phone purely for “sealant disasters.” I’ve seen putty harden to concrete inside chrome pop-ups and silicone peel like sunburn on a cheap plastic bath. Last spring I re-sealed 42 basements after Storm Kathleen; half the leaks came from the wrong product in the wrong place. When I write about putty versus silicone, I’m not repeating manufacturer leaflets—I’m recalling the £3,200 insurance claim I filed for a client who mixed them up.
Quick Answer: When to Grab What
1. Basket strainer, metal waste, no movement → Plumber’s putty (Oatey 31190, £6.50 at Toolstation)
2. Plastic waste, tiled over, slight flex → Neutral-cure silicone (Everbuild Everflex 500, £4.80 cartridge)
3. Underwater joint, submerged pump union → 100% silicone (Dow Corning 785+, £9.40)
4. Fireplace surround, gas pipe escutcheon → Putty only (silicone fumes at 60 °C)
Plumber’s Putty: Old-School, Still Relevant
Plumber’s putty is basically china clay whacked with linseed oil. Roll a worm, press, trim—job done. It never cures, so if you knock the waste later it’ll re-seat itself. That flexibility is why I still use it on thick enamel Belfast sinks where the basket flange sits proud. A tub of Hercules 75350 costs £5.20 and will seal twenty basket strainers; leftovers last three years if you keep the lid tight.
Downsides? It stains marble, granite and any porous stone. I once watched a £1,800 Carrara worktop bloom with a permanent grey ring because the fitter “forgot” putty bleeds oil. It’s also useless on plastics: the oil attacks ABS and polypropylene, turning them brittle. And if you over-tighten the waste, the putty squishes out completely, leaving a paper-thin skin that weeps within months.
Silicone Sealants: The Modern Flexible Shield
Silicone is a room-temperature-vulcanising rubber. Once the acetic acid (smells like vinegar) or alcohol (neutral cure) flashes off, you’ve got a waterproof gasket that stretches 25%. I use SikaSan 600 (£7.90) for plastic wastes; it sticks to PVC like gum to a shoe and handles the micro-movement when kids slam the plug chain. For submersible pumps I step up to Dow 732, rated for continuous water contact and 150 °C.
The catch is surface prep. Any soap scum, lime-scale or fingerprint oil creates a bubble trail. I wipe joints with Everbuild Wonder Wipes (£4.40 for 100) then hit them with Kärcher steam for five seconds. Miss that step and the silicone skins but stays wet underneath—classic “zipper failure” you can peel off in one strip. Silicone also hates solvent-based putty; if remnants of the old stuff remain, the new bead turns into cottage cheese within days.
Side-by-Side Comparison
| Feature | Plumber’s Putty | Silicone |
|---|---|---|
| Cure time | None – stays pliable | 24 h skin, 72 h full |
| Movement tolerance | Poor – can extrude | Excellent – 20–25% |
| Temperature limit | ~60 °C before oil bleed | 150–260 °C (product dependent) |
| Plastic compatibility | NO – attacks ABS/PVC | YES – with neutral cure |
| Stone staining | YES – oil wicks | NO – once cured |
| Removability | Finger roll, wipes clean | Blade scraper + solvent |
| Cost per joint | £0.25 | £0.60 |
| Lifespan | 5–20 years (dry location) | 10–40 years |
Real-World Scenarios I Face Every Month
Scenario 1: Retrofit stainless sink in oak worktop
Customer wants the full farmhouse vibe. Oak moves 3mm seasonally. Putty would pump out in six months, so I bed the flange with a 3mm silicone bead and add a second bead underneath for insurance. Eight years on, still bone-dry.
Scenario 2: Rental flat, plastic bath waste
Landlord insists on “cheapest fix.” I use putty because it’s £3 less and he’ll sell in two years. Thirteen months later the bath drops 1mm when tenants overflow it—putty ring fractures, water hits the ceiling below. I re-do it with Geocel 201 and charge him double.
Scenario 3: Outside drinking fountain, Staffordshire winter
Temperature swings from −5 °C to 12 °C in 24h. Putty goes rock hard, cracks. I switch to CT1 (hybrid polymer, £11.99) that stays flexible to −40 °C. Fountain stayed sealed through three freeze-thaw cycles.
Cost Breakdown for a Standard Kitchen Sink
- Putty route: 1lb tub Oatey £6.50 → 20 uses = £0.33
- Silicone route: 1 cartridge Everflex £4.80 → 8 uses = £0.60
- Labour to re-do failed job: £95 call-out + £25 materials
Moral: saving 27p can cost £120 if you guess wrong.
Common DIY Errors (and How I Fix Them)
1. “More is better” globs – Putty squishes into the waste tail, blocks flow. I snake it out with a bottle brush.
2. Silicone on top of old putty – Oil contaminates the bead. I razor-scrape both, clean with methylated spirit, re-seal.
3. Finger-smoothing silicone without tool – Leaves micro-grooves. I cut a 45° nozzle and use a Tajima profiling tool (£12) for a perfect fillet.
4. Using bath silicone (fungus-proof) – It contains biocides that taint drinking water. I swap to WRAS-approved Dow 785.
Safety Considerations and Legal Requirements
Water regulations are tighter than people think. Any sealant in contact with potable water must carry WRAS approval (UK) or NSF-61 (US). Standard hardware-store “bath & kitchen” silicone fails that test. If a letting agent’s inspection finds non-approved product, you can be forced to rip it out. For gas appliances, Building Regs Approved Document J bans combustible sealants within 50mm of a flue spigot—putty wins here because it’s inert clay.
Warning: Using acid-cure silicone on copper pipes → creates acetic acid pockets → pinholes in 18 months → flood. Always specify neutral cure near metal plumbing.
FAQ
Can I put silicone over plumber’s putty?
No. Oil bleeds through, silicone turns to Swiss cheese. Strip both, clean, start fresh.
How long before I can run water?
Putty: immediately. Silicone: 6h cautious, 24h full cure. I tell clients to wait overnight if they’re unsure.
Does colour matter?
Only for looks. Clear shows mistakes, white hides them, almond matches vintage porcelain.
Is putty food-safe?
Not certified. Use WRAS silicone if the sink drains to a coffee machine or ice maker.
Why does my silicone go black?
You used bathroom grade in a kitchen. Fungicide washes out, soap feeds mould. Swap to sanitary grade with higher biocide load.
Can I speed up curing?
Heat gun or hair-dryer on low helps skin, but full cure is chemical; you can’t rush 24h.
Final Take
Pick putty for old-school metal wastes that never move; choose silicone for plastics, stone tops, or anything that shivers when the seasons change. The upfront 30p saving on putty evaporates the moment you pay a pro £120 to redo the job. Buy the right tube, clean the joint like you’re about to eat off it, and your next under-sink peek will be for lost sponges, not dripping horror.
Mike Rodriguez