Knowledge Base & Guides

Choosing Bedding Materials for your Hatches: Butyl vs. Foam Tape vs. Polyurethane

Summary

  • Butyl tape. It seals exceptionally well, stays pliable for decades, and makes future removal relatively painless. Doesn’t stand up to solvents. Make sure to retorque after ~24 hours.

  • Inseal foam tape (factory‑style). Clean surface, apply tape to the hatch flange, and fasten. No next‑day retorque. When it’s time to remove, expect to use a mild adhesive remover for the tape’s acrylic adhesive.

  • Marine polyurethane sealants (e.g., Sikaflex® 291/292, 3M™ 4200). They bond parts together and can mask poor fastener practice—but they make later service difficult and risk damage during removal. On cored decks, especially foam core, proper core closeouts mean extra adhesion usually isn’t needed.

Why Bedding Choice Matters

Your hatch’s bedding has two jobs: (1) keep water out of the joint between the hatch flange and the deck, and (2) allow maintenance down the road. Hatches need periodic service—gaskets wear, lenses get replaced, hardware needs attention. Choosing a bedding that you can’t remove later turns a routine job into an expensive project.

Flatness requirement: Gebo hatches, similar to most major brands, are designed to be installed on a flat surface within 1 mm of curvature. If the mounting surface isn’t flat, fair it before installation. A distorted frame can compromise sealing and void your effort.

The Usual Suspects

  • Polyurethane sealants (e.g., Sikaflex® 291/292, 3M™ 4200)

  • Butyl tape (high‑quality marine butyl)

  • Inseal foam tape (factory‑style gasket tape for hatch flanges)

Polyurethane Sealants (Sikaflex® 291/292, 3M™ 4200)

Pros

  • The only option in this group that adds meaningful adhesion between hatch and deck.

Cons

  • Nightmare-inducing removal—often leads to damage to the gelcoat, paint, or the hatch frame during service.

Notes

While this may seem like a great option when the deck surface may not hold fasteners well, such as with a foam core, the reality is that are better ways to install.  For all cored decks, any screw or bolt holes should be prepared with a “core closeout.” This is the process of overdrilling the size of the hole, filling with appropriate marine epoxy, and drilling a new hole for the fastener.  
Core closeouts prevent core crushing, enable proper torque on fasteners, and block water migration. Once you’ve done this correctly, extra polyurethane adhesion offers little benefit for a hatch—and hurts serviceability.
Recommended reading: Steve D’Antonio’s “Installing hardware in cored composites” article featured in Professional BoatBuilder Magazine
Steve explains a great process for core closeouts with useful imagery to match.

Butyl Tape 

Pros

  • Remains pliable for decades; accommodates minor movement and self‑heals small shifts.

  • Future removal is simple—great for routine hatch service.

Cons

Solvents are butyl’s kryptonite. Avoid installations near fuel fills or where solvent based cleaners, like acetone or mineral spirits, are routinely used.

Pro Tips for Install

  • Retorque after ~24 hours. Butyl will creep a touch under compression. Snug it again the next day.

  • Countersink each fastener hole slightly on the deck side. Before setting the hatch, roll a small “butyl donut” and place it around each hole under the flange. This creates an O‑ring effect that seals the fastener shank and threads.

  • Bolted joints: Insert the bolt through hatch and deck first. Hold the head still and tighten only the nut from below. Spinning the fastener can grab the butyl and pull it out of adjacent areas without you noticing.

  • Screws (when bolts aren’t possible): There’s no perfect way to avoid some twisting of the butyl as screws cut threads. Work slowly and keep pressure even.

Inseal Foam Tape (factory‑style and Gebo recommendation)

Pros

  • Fastest option. Clean, stick the tape to the underside of the hatch flange, place, and fasten.

  • No next‑day retorque needed—foam maintains compression.

  • Clean install. The tape forms a continuous gasket.

Cons

The only (very minor) con is that the tape’s acrylic adhesive may need a household adhesive remover during future removal.

Notes

The experts at the Gebo factory do not recommend any additional sealer for the holes, as long as the tape goes over the holes and is intact.  They mention, however, that some boat builders do add additional sealer to the holes per their preferences.

Surface prep & mounting best practices

  • Core Closeouts for fasteners on cored decks - Recommended reading: Steve D’Antonio’s “Installing hardware in cored composites” article featured in Professional BoatBuilder Magazine.  Steve explains a great process for core closeouts with useful imagery to match.

  • Prevent galvanic trouble & make future service easier: Apply a thin smear of Tef‑Gel (or similar PTFE anti‑seize) to the underside of stainless screw heads where they bear on the aluminum hatch frame. This reduces corrosion and eases removal later.

  • Flat, clean, dry, degreased surfaces only. Wipe with a fast‑evaporating, compatible solvent and allow to flash off completely before applying tape or sealant.

  • Seal exposed core in the cut‑out hole. Any raw core should be sealed with epoxy to prevent water ingress.

  • Dry‑fit first. Confirm fastener length, hole alignment, and that the hatch sits flat (≤ 1 mm tolerance).

  • Re‑check after 24 h (butyl only). For foam tape and polyurethane, a single controlled torque is usually sufficient per the product’s instructions.