Butyl Tape Application for Boat Windows and Hatches

The right tool for this job

Butyl Tape RV and Boat Window Seal Sealant

Peel the liner and press the putty tape into place. The 36 foot roll is 3/4 inch wide and 1/8 inch thick, in black, gray, or white.

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Butyl tape application is the fix for the slow drip that lands on the cabin sole after every rain, or the damp streak under a hatch that never quite dries. The tape seals the joint between a window or hatch frame and the boat as the fasteners pull the frame down. Done right, it compresses into a gasket between the frame and the boat. Done wrong, it squeezes out, bridges a gap, or leaves a channel that water follows straight inside.

This guide covers prep, placement, corners, fastener tightening and common mistakes that cause leaks. The tape referenced throughout is Better Boat Butyl Tape RV and Boat Window Seal Sealant, a putty tape 1/8 inch thick and 3/4 inch wide on a 36 foot roll, in black, gray or white.

Why Butyl Tape Works for Boat Windows and Hatches

Butyl, pressed between a frame and the cabin side, fills small voids, gelcoat ripples and screw holes, then holds that seal as long as the fasteners keep it under compression.

• No mixing. You peel the liner and press it in place.
• Heat tolerant. The Better Boat tape is rated to 290 degrees F.

The product listing describes use on windows, seams, fixtures, vents, trim and gaps. On a boat, that makes it a fit for opening ports, fixed windows, deck hatches, vents and trim pieces that are screwed or bolted in place.

Tools and Surface Prep Before You Start

Clean, dry, residue free surfaces are what make butyl seal.

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Microfiber Cloth Set 12Pk

Twelve large 16 by 16 inch microfiber cloths with dual sided pile and silk covered edges. Machine washable and reusable for wiping and drying surfaces.

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Gather these before removing the window.

• Plastic scraper or putty knife (metal blades gouge gelcoat and aluminum)
• Isopropyl alcohol and lint free cloths
• Sharp utility knife for cutting tape and trimming squeeze out
• The correct screwdriver or wrench for the frame fasteners
• Painter's tape to mark fastener holes, if the frame is large

Then prep the surfaces in order.

1. Remove the frame and scrape off all old sealant from both the frame flange and the opening.
2. Wash the area around the opening to lift salt, grime and wax. A cleaner from the Better Boat hull and exterior cleaners range handles this step.
3. Wipe both mating surfaces with isopropyl alcohol on a clean cloth from the sponges, cloths and towels collection. Keep solvents off acrylic or polycarbonate lenses unless the window maker approves them.
4. Let everything dry completely. Moisture trapped under butyl becomes the start of the next leak.

How to Apply Butyl Tape to a Boat Window or Hatch

Apply the tape to the frame flange, not the boat, then set the frame, snug the fasteners evenly and let the butyl compress before the final tightening. Working on the frame keeps the tape straight and lets you see every fastener hole.

1. Pick the color. Choose black, gray or white to match the frame or surrounding gelcoat, since a thin bead of squeeze out stays visible.
2. Lay the frame face down on a padded surface so the flange faces up.
3. Start at the top center of the frame. Peel back a short section of liner and press the tape onto the flange, centered between the outer edge and the fastener holes.
4. Press, do not stretch. Unroll and press the tape down with your fingers as you go. Stretched tape thins out and shrinks back, leaving gaps.
5. Follow the flange around every corner in one continuous piece (corner technique is below).
6. Join the ends at the top where you started, then remove the remaining liner.
7. Set the frame in the opening and start every fastener by hand so nothing cross threads.
8. Snug the fasteners in a crisscross pattern until the butyl begins to squeeze out evenly around the edge. Stop there and leave it to settle.
9. Retighten after the butyl has settled, ideally on a warm day, snugging each fastener a little more in the same crisscross order.
10. Trim the squeeze out with a plastic scraper held flat against the surface once the final tightening is done.

Handling Corners, Joints and Fastener Holes

Corners, end joints and fastener holes are where butyl leaks start. Handle each one deliberately and the rest of the joint takes care of itself.

Corners

Run the tape around corners in one piece instead of cutting it. On a rounded corner, ease the tape around the curve and press it down. On a tight corner, pinch the tape into a small fold and press the fold flat so no gap opens at the bend. A cut corner gives water two seams to find.

Where the Ends Meet

Butt or slightly overlap the two ends and press them together until they merge into one mass. Place this joint at the top of the frame, never the bottom, because water collects at the low edge and works into any weak spot. Wrapping the frame in one continuous run keeps the joint count to one.

Fastener Holes

Every screw is a path for water, so seal the holes as well as the perimeter. Keep the main strip on the outboard side of the fastener line, then twist a small piece of butyl around each screw shank just under the head before you drive it. The butyl fills the hole and seals the screw head as it tightens.

Butyl Tape Compared With Other Marine Sealants

Curing sealants are an alternative to tape, so check each product's label for its intended use and bond strength.

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Marine Sealant & Adhesive Caulk

Seal leaks, seams, portholes, and repairs above or below the waterline once fully cured.

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Some window makers specify a particular sealant for their frames or lenses. Check the manufacturer's instructions before switching. For jobs that need a curing product, browse the Better Boat sealants, tape and repair collection.

Common Butyl Tape Mistakes That Cause Leaks

Cranking the frame down at once squeezes out the tape and leaves no gasket between the frame and the cabin side.

Overtightening also stresses acrylic and polycarbonate lenses, which can crack around the screw holes. Snug fasteners firmly, not as hard as they will go, and follow the window maker's guidance where it exists.

Check bedded windows and hatches at the start of each season.

Frequently Asked Questions

Does butyl tape ever dry or cure after you apply it?

Better Boat Butyl Tape is a putty style tape that you peel and press in place, with no mixing.

Should you put butyl tape on the window frame or on the boat?

Put butyl tape on the window frame flange. Working on a frame lying flat lets you keep the tape straight, see every fastener hole and control the corners, which is much harder on a vertical cabin side.

Why is butyl tape oozing out from under my boat window?

Some squeeze out is normal and shows the joint is compressed. Trim it flush with a plastic scraper once the fasteners are fully snug.

Can you use butyl tape below the waterline on a boat?

Use butyl tape for the jobs its listing describes, such as windows, hatches, vents, trim, seams and gaps. For through hull fittings or anything below the waterline, use the sealant the fitting or boat manufacturer specifies.

Do you need to retighten screws after butyl tape application?

Yes, retighten the screws after the butyl settles, snugging each one a little more in the same crisscross order and following the window maker's guidance.

What does butyl tape not stick to?

Butyl tape grips poorly to surfaces that are dirty, wet, or coated with old sealant residue. Scrape the surface fully, wipe it with isopropyl alcohol, and let it dry before pressing the tape on. Better Boat Butyl Tape is listed to bond to EPDM rubber, fiberglass, aluminum, vinyl, ABS plastic, and wood. The fasteners hold the frame while the tape seals it.

What are the downsides of butyl tape?

The screws or bolts have to hold the frame, and the fasteners need to be snugged again after the tape settles. Visible squeeze out also has to be trimmed with a plastic scraper. It is not the right choice for through hull fittings or anything below the waterline, where the fitting or boat manufacturer's specified sealant should be used.

Does butyl tape harden over time?

Butyl tape is a putty style tape with no mixing step. Check the manufacturer's instructions for how it holds up over time.

The Bottom Line

Good butyl tape application comes down to clean surfaces, tape on the frame flange, continuous corners, the end joint at the top and patient two stage tightening.

Better Boat Butyl Tape RV and Boat Window Seal Sealant gives you 36 feet of 1/8 inch by 3/4 inch putty tape, rated to 290 degrees F, in black, gray or white to match your frames. Peel the liner, press it in place and seal the leak at its source.

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