Marine 4000 UV vs 5200: Which Sealant Should You Use?
If you've ever stood in the marine aisle staring at two tubes of sealant wondering what the difference is between 4000 UV vs 5200, you're not alone. It's one of the most common questions among boat owners, and picking the wrong one can mean a leaking hull fitting — or a deck fitting you can never remove without a heat gun and a lot of frustration. The short answer: 4000 UV is for above-waterline applications where UV exposure matters and future removal is possible. 5200 is for permanent, below-waterline bonds you never want to touch again. Here's how to know which one your project actually needs.
Understanding 4000 UV vs 5200: What Makes Them Different
Both 4000 UV and 5200 are polyurethane-based marine sealants, which means they share the same core chemistry — flexible, waterproof, and far more tenacious than silicone. But polyurethane sealants aren't all equal, and the formulation differences between these two products are significant.
5200 is a one-part, moisture-curing polyurethane with an exceptionally aggressive adhesion profile. It bonds to fiberglass, wood, and metal with a grip that approaches structural adhesive territory. Once it cures fully — a process that takes 5 to 7 days — the bond is essentially permanent. Boatyards use it for hull-to-deck joints, keel bolts, through-hulls, and any fitting where catastrophic failure would mean water in the bilge. The permanence is the point.
4000 UV is also a polyurethane, but formulated with two key modifications: UV stabilizers to prevent the sealant itself from breaking down under sun exposure, and a somewhat less aggressive adhesion formula that allows for future disassembly without destroying the substrate. It also remains slightly more flexible once cured, which matters for deck hardware that experiences constant vibration and thermal cycling.
The UV stabilizers in 4000 UV are not a minor detail. Standard polyurethane sealants (including 5200) will chalk, harden, and crack when exposed to prolonged UV radiation. If you use 5200 above the waterline on a deck fitting that bakes in the sun, you'll see it fail within a season or two. 4000 UV is engineered specifically for that environment.
The Real Decision: Do You Ever Need to Remove It?
This is the question that actually determines which sealant you need, and most boaters don't think about it until they're trying to pull off a fitting and taking fiberglass with it.
Use 4000 UV when:
- The fitting is above the waterline (deck hardware, port lights, hatches, cleats, navigation lights)
- There's significant UV exposure
- You may need to remove or replace the fitting in the future
- You want a watertight seal that's still serviceable
Use 5200 when:
- The fitting is below the waterline (through-hulls, thru-deck fittings, shaft logs)
- You need maximum bond strength and water exclusion
- Removal is not a consideration — this is a permanent installation
- Structural integrity is paramount
One practical point worth knowing: professional boatyards keep both on hand and use them carefully. A through-hull installed with 5200 is nearly impossible to remove without risking damage to the hull laminate. A seasoned marine technician will sometimes use 4200 (a medium-strength version) in situations where they want more bond than 4000 UV but less permanence than 5200. Berkland's 4000 UV formula covers the above-waterline, removable-bond use case — the most common application for DIY boat maintenance.
Side-by-Side Comparison
| Feature | 4000 UV | 5200 |
|---|---|---|
| Primary use | Above-waterline, deck hardware | Below-waterline, hull fittings |
| UV resistance | Yes — UV stabilized formula | Limited — can degrade in sun |
| Removability | Yes — can be cut free with effort | No — nearly permanent bond |
| Bond strength | High | Extremely high (structural) |
| Flexibility when cured | High | Moderate |
| Full cure time | 5–7 days | 5–7 days |
| Works on fiberglass | Yes | Yes |
| Works on wood | Yes | Yes |
| Works on metal | Yes | Yes |
| Best for port lights | Yes | No |
| Best for through-hulls | No | Yes |
How to Apply Marine Sealant Correctly
The right sealant choice only matters if application technique is correct. Both 4000 UV and 5200 require the same prep steps:
Surface preparation is non-negotiable. Both sealants need clean, dry, grease-free surfaces. Wipe down with acetone or isopropyl alcohol and let it flash off completely before applying. Any contamination — wax, silicone residue, oil — will cause adhesion failure.
Tape off the area. Polyurethane sealant is extremely difficult to remove from gelcoat once cured. Painter's tape is your friend. Apply it before you start, and pull it while the sealant is still wet (within the first 10-15 minutes) for a clean edge.
Bed hardware properly. For deck hardware, apply sealant to the underside of the fitting and around the fasteners. When you tighten down, squeeze-out should appear around the perimeter — that's how you know you have full coverage. Wipe the excess immediately.
Temperature matters. Both sealants require temperatures above 40°F to cure properly. Cold weather slows moisture-cure polyurethane significantly, and below freezing it can stop entirely. Allow extra cure time in cool conditions, and never apply to a surface with frost or condensation.
Don't rush the cure. Both products reach a skin in 24-48 hours but aren't fully cured for 5-7 days. Launching before full cure means the bond hasn't reached full strength.
What About Silicone?
You'll sometimes see boaters using silicone sealant as a lower-cost option. For most marine applications, silicone is the wrong choice — it lacks the adhesion strength of polyurethane, it's not as waterproof under pressure, and critically, any surface that has been contaminated with silicone will resist adhesion from future sealants. Silicone is nearly impossible to fully remove from a porous surface. The only place silicone makes sense on a boat is for interior applications where there's no structural requirement and no plan to ever re-seal.
What We Recommend
For most boat owners doing their own maintenance, you'll eventually need both of these sealants — they serve different purposes and there's no universal substitute. That said, deck hardware projects come up far more often than through-hull work, which means 4000 UV tends to get more use in the average boat owner's toolkit.
Berkland 4000 UV Marine Sealant — UV-stabilized polyurethane for above-waterline applications
- UV stabilizers prevent surface degradation and chalking in sun-exposed installations
- Removable bond — you can take deck hardware off years later without destroying the substrate
- Stays flexible after cure, handling the constant vibration and thermal cycling of deck fittings
For permanent below-waterline work, our 5200 FC Marine Sealant delivers the structural-grade bond that through-hulls and hull fittings require — the same polyurethane chemistry trusted by professional boatyards.
Buy 5200 FC Marine Sealant on Amazon →
Frequently Asked Questions
Can I use 5200 above the waterline instead of 4000 UV?
You can, but it's not recommended for two reasons: 5200 lacks UV stabilizers and will degrade faster in sun-exposed locations, and the permanent bond makes future hardware removal very difficult without damaging the substrate. For above-waterline deck hardware, 4000 UV is the right tool.
How long does 4000 UV take to cure?
4000 UV reaches an initial skin within 24-48 hours at room temperature (above 60°F) and reaches full cure in approximately 5-7 days. Humidity actually helps moisture-cure polyurethane — a slightly humid environment speeds the process. Cold temperatures slow it significantly.
Can I apply 4000 UV over old sealant?
No. For a reliable bond, you need to remove the old sealant completely and apply to bare, clean substrate. Applying over existing sealant — even the same product — creates a bond to the old layer rather than to the surface itself, and that bond is only as strong as the old sealant's adhesion.
What's the difference between 3M 4000 UV and the Berkland version?
Both are UV-stabilized, one-part polyurethane marine sealants formulated for above-waterline applications with comparable chemistry and performance. Berkland's 4000 UV Marine Sealant offers the same working properties at a more accessible price point.
Does 5200 work on aluminum?
5200 adheres to aluminum, but aluminum requires a primer for maximum adhesion — particularly in submerged or wet environments. Clean the surface thoroughly, apply an adhesion promoter designed for aluminum, and allow it to tack before applying the 5200. Skipping the primer step is the most common cause of 5200 failure on metal substrates.
You might also like:
- 3M 5200 Marine Sealant: Complete Buyer's Guide (2026)
- Marine Sealant Cure Time: How Long Before It's Waterproof and Load-Bearing
- Marine Sealant for Pontoon Boats: What to Use Above and Below Waterline
Related reading:
- The Complete Guide to 3M 5200 Marine Sealant
- Best Waterproof Sealant for Boats
- Marine Sealant Cure Time: What to Expect
Shop this product: Marine Sealant on Berkland Goods