How to Choose the Right Sealant for Every Construction Job
Why Sealants Matter More Than You Think
You know what’s wild?
People will spend £10,000 on the perfect roof, hire the best trades for a flawless fit-out, and then…
They grab the cheapest tube of sealant from the shelf — and wonder why the job fails 3 months later.
It’s not a mistake. It’s a misunderstanding.
Because when you look at it, sealants don’t feel like a big deal. They’re the finishing touch. The afterthought. The “grab-one-while-you’re-at-the-merchant” moment.
But here’s the truth:
Sealant choice can make or break your build — literally.
A cracked window line because the silicone couldn’t hack the cold.
A leaking bathroom because acrylic sealant couldn’t hold back the moisture.
A failed fire rating because you used the wrong filler in a wall penetration.
And the worst part?
Most builders don’t even realise they’ve picked the wrong product — until the call-back costs hit.
That’s why this guide exists.
We’re going to cut through the nonsense, the jargon, and the 300 varieties of “multi-purpose” sealants no one really understands. You’ll get:
- A breakdown of the different types of sealants used in UK construction (and which one to use where)
- What to check for when choosing a weatherproof or fire-rated sealant
- How to avoid compatibility issues with substrates like concrete, timber, glass or metal
- Simple application tips to ensure it holds — first time, every time
- And how to save time and money by getting it right from the start
No fluff. No affiliate “Top 10” lists. Just real-world advice backed by construction know-how.
By the end of this guide, you won’t just know which product to pick.
You’ll know why it works, where it fails, and how to make sure it sticks — literally and figuratively.
Oh — and if you’re ready to browse construction-grade adhesives and sealants now, check out our full range here:
Sealants & Adhesives
Let’s get into it.
Types of Construction Sealants & Where to Use Them (Without Guesswork)
Here’s the problem with most sealant advice online:
It either gives you a science lesson on polymers…
…or tells you to “just use silicone for everything.”
But the truth is, every sealant has a strength — and a breaking point.
Use the wrong one, and you’re looking at failed adhesion, leaks, or movement cracks in a few months. So let’s simplify it.
1. Silicone Sealant (Flexible + Waterproof)
Best for: Bathrooms, kitchens, glass, metal, uPVC
Avoid if: You’re painting over it — paint doesn’t stick to silicone
Why it works:
Silicone is flexible, weatherproof, and doesn’t degrade with moisture. That’s why you see it in shower trays, window frames, and glass-to-metal junctions.
But not all silicone is created equal.
- Low modulus = more flexible → better for joints that move (expansion joints)
- High modulus = less movement, but stronger adhesion → great for rigid sealing (glass corners)
If you’re sealing joints around tiles or glass, this is your go-to.
Pro Tip: For a crystal-clear finish, use masking tape and remove before the silicone skins. No smudges, no mess.
2. Acrylic Sealant (Paintable but Rigid)
Best for: Skirting boards, door frames, internal gaps before decorating
Avoid if: It’s exposed to water — acrylic isn’t waterproof
Why it works:
It’s a decorator’s dream. You can slap it into a crack, smooth it off, and paint over it within the hour. Great for internal use.
But here’s the catch: it doesn’t flex well. If there’s expansion/contraction (like timber around windows), it’ll crack over time.
So think: interior finishing, not bathrooms or exteriors.
3. Polyurethane (PU) Sealant (Heavy-Duty Performance)
Best for: Concrete joints, external expansion joints, roof sealing, structural jobs
Avoid if: You need UV stability — it can yellow or degrade in sunlight if unprotected
Why it works:
This stuff means business. It bonds like glue, fills joints, flexes with movement, and handles serious loads.
Think bridges, car parks, precast concrete, structural glazing.
It’s commonly used in commercial construction, roofing, and external movement joints.
Heads-up: Polyurethane is sticky, aggressive, and a pain to clean off tools — so mask areas well and wear gloves.
4. MS Polymer (Hybrid) Sealants (The New All-Rounder)
Best for: General-purpose construction sealing, multi-surface jobs, kitchens, bathrooms
Avoid if: You’re on a super-tight budget — hybrids cost more, but they’re worth it
Why it works:
MS Polymer combines the best of both worlds: the flexibility and waterproofing of silicone, with the paintability of acrylic — and it bonds to nearly everything.
Ideal if you’re sealing multiple materials (brick to glass, wood to metal) or want one product for the whole job.
5. Bituminous Sealant (Specialist Waterproofing)
Best for: Roofs, gutters, damp-proofing, tanking
Avoid if: You’re sealing anything indoors — it smells and can stain surfaces
Why it works:
Bitumen-based sealants are designed for extreme waterproofing. They stick to damp surfaces, fill roof cracks, and resist weather abuse.
Want to browse roof sealing products? Check out our Roofing Category
6. Fire-Rated Sealants (Passive Fire Protection)
Best for: Wall penetrations, electrical chases, fire compartment joints
Avoid if: You’re sealing standard internal finishes — fire-rated is overkill for basic jobs
Why it works:
These sealants expand when exposed to heat, blocking the spread of fire and smoke. Essential for fire doors, cable penetrations, and high-risk areas.
Check the fire resistance rating (EI60, EI120, etc.) before applying.
Comparison Table – Sealant Type vs. Best Use
| Sealant Type | Ideal Use Cases | Paintable? | Waterproof? | Flexible? |
| Silicone | Kitchens, bathrooms, windows | ❌ | ✅ | ✅ |
| Acrylic | Internal joints, skirtings, door frames | ✅ | ❌ | ❌ |
| Polyurethane (PU) | Expansion joints, structural concrete | ❌ | ✅ | ✅✅ |
| MS Polymer | Multipurpose, all surfaces | ✅ | ✅ | ✅ |
| Bitumen-Based | Roofs, damp-proofing, gutters | ❌ | ✅✅ | ✅ |
| Fire-Rated | Fire walls, pipe entries, M&E works | ✅ | ✅ | ✅ |
How to Choose the Right Sealant for Your Construction Job
You’ve seen the options.
But how do you actually choose the right sealant for your job?
Because let’s be honest…
Even experienced builders sometimes play sealant roulette: “This one looks heavy-duty. Chuck it in.”
The result?
Peeling edges. Cracks after a frost. A call-back from the client. And a whole lot of cursing.
Let’s fix that.
Here’s the step-by-step checklist that takes out the guesswork — and makes sure you grab the right sealant the first time, every time.
Step 1: What Surface Are You Sealing? (Compatibility Comes First)
Not all sealants bond well to all materials.
| Surface Type | Best Sealant Choice |
| Glass | Silicone or Hybrid (MS Polymer) |
| Concrete | Polyurethane (PU) or Bitumen-based |
| Timber | Acrylic or MS Polymer |
| Plastic/uPVC | Silicone or Hybrid |
| Metal | Hybrid or Polyurethane |
Real Talk: If you’re working across multiple materials (e.g., brick to glass), go for an MS Polymer Sealant — it’ll bond to both and won’t degrade in sunlight.
Step 2: Will It Be Exposed to Water or Weather?
If the answer is yes — stop right now and check the waterproof rating.
This is where cheap acrylic fails fast. You need permanent elasticity and water resistance.
| Environment | Recommended Sealant |
| Internal (dry) | Acrylic or Paintable MS Polymer |
| Wet Area (bathroom) | Silicone or Hybrid |
| External (facade) | PU Sealant or Hybrid |
| Roofs & Gutters | Bitumen Sealant |
Need weather resistance? Shop External-Grade Sealants from MPS.
Step 3: Does the Joint Move? (Flexibility Matters)
Movement is where most sealants fail. Expansion, vibration, heat changes — all stress the joint.
If you’re sealing:
- Expansion joints
- Window frames
- Structural elements
…then you need low modulus, flexible sealants.
Go for:
- Polyurethane for heavy-duty flexibility
- MS Polymer for clean, all-surface jobs
- Low-modulus silicone for windows and doors
Avoid:
- Acrylic (it cracks with joint movement)
Step 4: Will You Be Painting Over It?
This is where most trades slip up.
Silicone is not paintable. Even with “paint-over” tricks, most emulsions will flake or bead off.
If you need a smooth finish and painted edge (e.g., skirting board to wall), use:
- Acrylic Sealant
- Paintable MS Polymer (for more flexibility)
Want perfect lines? Mask edges before sealing and paint after 24 hours for a flawless finish.
Step 5: Is Fire-Rating or Acoustic Proofing Required?
Modern builds often need to meet fire compartmentation or soundproofing standards.
In this case, standard sealant won’t cut it. You need:
- Fire-rated sealant (certified EI60 or EI120)
- Acoustic-rated sealant (typically hybrid or specialist acrylic)
Use these for:
- Sealing around pipe penetrations
- M&E service voids
- Lift shafts, stairwells, and risers
- Partition walls in office or hotel fit-outs
MPS can source fire-rated sealants on request. Contact us here for bulk orders.
Step 6: What’s the Application Like? (Tooling, Skin Time, Cleanup)
Some sealants are easy to work with. Others feel like you’re battling glue with a spoon.
Here’s what to check on the label (or spec sheet):
- Skin time – how fast it sets (faster = less clean-up time)
- Cure time – when it reaches full hardness
- Tooling time – how long you’ve got to smooth it before it starts setting
- Clean-up – some need solvent, some wipe with water
If speed matters (e.g., in cold weather), grab fast-skinning MS Polymer.
Tip: Always keep a rag, bucket, and sealant wipe handy. Nothing ruins your finish like a finger drag or dirty joint.
Quickfire Summary: Sealant Match Guide
| Job Type | Ideal Sealant |
| Bathroom sealing | Low-modulus silicone |
| Roof crack repair | Bitumen-based sealant |
| Window frame joints | Hybrid or low-modulus silicone |
| Timber skirting (painted) | Acrylic |
| Concrete expansion joint | Polyurethane |
| Fire-rated wall penetration | Fire-rated acrylic or hybrid |
| Multi-surface sealing | MS Polymer |
Common Sealant Mistakes (That Cost You Time, Money, or Compliance)
Here’s the truth…
You can have the best product, the perfect surface, and even the steadiest hand — and still mess up a sealant job.
How?
By skipping the little things that make or break performance.
Below are the most common mistakes we see — along with dead-simple ways to avoid them.
1. Using the Wrong Sealant for the Job
We covered this earlier, but it’s the #1 fail across UK sites.
Silicone in a paintable joint. Acrylic in a wet area. PU where UV resistance is essential.
Fix: Always double-check surface compatibility, exposure level (moisture, movement, UV), and whether you need it to be paintable.
Explore all Sealants & Adhesives
2. Not Prepping the Surface
Think of sealant like a tattoo — once it’s in, it’s hard to fix.
Sealant applied to dusty, damp, or greasy surfaces will peel or bubble. Full stop.
Fix:
- Clean the area
- Dry thoroughly
- Degrease (especially uPVC or metals)
- Prime, if required (check the spec sheet)
MPS stocks cleaning wipes and prep solutions
3. Forgetting to Use a Backing Rod
This one’s HUGE in expansion joints.
When the joint is too deep, and you fill it fully with sealant, it’s more likely to crack. Why? Because it can’t flex in the middle.
Fix:
Use a closed-cell backing rod to fill the void first. It supports the sealant, improves flexibility, and saves product too.
4. Applying Too Thick or Too Thin
Sealant that’s too thin? It peels.
Too thick? It skins on the outside but stays wet inside (and traps moisture).
Fix:
Always check application thickness on the label. If in doubt:
- Interior joints → 3–5mm
- Exterior joints → 8–10mm
- Movement joints → follow manufacturer guidance (often 10mm max)
5. Skipping Tooling (or Over-Tooling)
Tooling is when you smooth the joint with a sealant shaper or finger.
It ensures strong contact with both sides. But too many go overboard or skip it entirely.
Fix:
- Use a proper sealant shaper
- One smooth pass — no overworking
- Wipe excess with a damp cloth immediately
6. Not Reading the Cure Time
You’d be shocked how many re-paint or pressure-tests before the sealant has cured.
Fix:
Cure time ≠ skin time. Always check full cure time (usually 24–48h). MS Polymer tends to cure faster than PU.
If applying in cold or damp weather, add 30–50% more time to be safe.
7. Ignoring Fire or Acoustic Requirements
Especially in commercial jobs — this is not optional.
If you don’t use certified sealant in required areas (e.g. M&E risers, walls, hotel ceilings), you could fail inspection or lose your warranty.
Fix:
Use EI-rated sealants (e.g. EI60, EI120), and keep the certs for your site file.
Need help sourcing specialist products? Contact MPS — we stock or order on demand.
Final Take: Sealant Isn’t “Just a Tube” — It’s Job Protection in a Cartridge
The next time you pick up a sealant gun, remember this:
You’re not just filling a gap.
You’re:
- Preventing water ingress
- Holding structures together
- Meeting fire codes
- Avoiding call-backs
- Saving yourself (or your client) thousands in rework
At MPS Construction Supplies, we’ve made it easy to get the right sealant, tools, and trade advice — without the guesswork.
Here’s where to start:
Shop Sealants & Adhesives
Browse Finishing Tools & Accessories
Ask Us for Expert Advice
We’re here for trades who don’t cut corners.
Let’s seal the job right — every time.
