Sealing Leaky Windows Properly
Paikea’s windows are quintessential Catana — curved, space-age looking, and about as impractical and expensive to replace as boat windows get. On a modest cruising budget, new windows were never really on the table. So the job became fixing what we had rather than replacing it. The crazing in the perspex is staying — until someone points us toward a magic product that buffs that out — but the squeaking and leaking were fixable, and we finally got around to doing it properly after living with it for two and a half years. The first six months on board, we told ourselves it was fine. It was not fine. It was a genuinely bad original install.
The product doing the work is an MS sealant — modified silica, a polymer blend that sits somewhere between a silicone and a polyurethane like Sika. It bonds like the urethanes without needing a primer, but stays flexible like a silicone, which is exactly what a window wants. It’s not a new idea to us — years ago, building an eight-metre sports boat in Australia, the windows kept falling out no matter what we tried, until this stuff went in and never gave us a problem again. We recently found out a major window manufacturer in New Zealand has switched to it too, for the same reason.
There are two versions worth knowing about: a standard one, and an “extreme tack” construction version that’s thick — hard to push through the gun, but genuinely useful for that reason. A thick bead makes it easy to build a proper bond line, and bond line thickness matters more than it looks like it should. You need real distance between the window and the boat for the sealant to move and stretch with the two surfaces. Close the gap up too tight and the shearing distance is high for the same amount of movement; give it more distance to work with, and the same 10mm of movement is spread across far more material. We bonded the windows in with the thick version for that reason, then went around and sealed the outside edges with the regular one.
One genuine advantage over polyurethane sealants like Sika: this stuff is naturally UV stable. Polyurethanes degrade fast in direct sun; this doesn’t. For a boat living outdoors full-time, that’s not a minor detail.
It wasn’t a flawless job. The glue skinned over faster than expected in the sun on a couple of edges, which meant it stopped being tacky before it properly bonded to the perspex — visible afterwards as odd light and dark patches where the bond hadn’t taken. That got sorted in the edge-fill pass, breaking through the skin to get the soft, gooey centre to actually stick. Screw holes, used temporarily to hold the windows in place overnight while the glue cured, got backfilled with the same sealant once the screws came out.
The low point of the job was a genuine mistake: working by torchlight at nine at night, one screw went in a touch too tight, and a small crack in one window grew overnight. Drilling a stop hole didn’t hold it — the crack jumped straight past the stop and ran clean through. It’s backfilled with the same MS sealant now and holding up fine, but it’s a real scar on an otherwise good job, and a reminder not to do delicate work at 9pm when you can’t see what you’re doing.
Where it landed: no more squeaks, no more leaks through the main window panels. There’s one smaller job still to chase — the seal where a hatch meets one of the windows looks marginal after a decent rain, so that’s next. But overall, the windows came out a long way better than what they replaced, and considering the state of the original install, we’re genuinely happy with it. Worth noting too — the sealant can be sanded back to a clean, matte finish with something like 300 grit if you want it looking sharp. We haven’t gotten to that yet. There are, as always, ten million other jobs ahead of it.
Related Reading
If you’re dealing with a different kind of leak or bond challenge on board, our composite hatches series covers similar bonding principles applied to structural composite work.