6 Lb. Density Expanding Pour Foam, 2 Part Polyurethane Closed Cell Liquid Foam for Boats, Dock Flotation, Carving, Soundproofing, Filling Voids, and Insulation (Quart Kit)

This worked well to fill a small void in a fiberglass boat step project I was working on. I am not sure how waterproof it would have been because I encapsulated it with epoxy before covering it with glass and Coosa board. One thing I had trouble with was figuring out how much to mix for the size of void. That information was provided but I way over estimated how much was needed. It was easy enough to cut off the excess with a hack saw. The image attached is "after" I made the adjustment to closer to the right amount. Making that calculation easier for smaller quantities would be nice. Along those lines, markers on the bottles for smaller batches would be very helpful. Nice product though.

This saved me having to rip out and re-mortar a tub. First time mortar bed for fiberglass acrylic tub and the mortar shrank leaving voids. This fills the voids perfectly, it’s incredibly solid now. In case you’re wondering as I was, this will go through a tube to get it exactly where you need it. I was afraid it’d expand in tube before exiting. I had it mixed and poured in funnel though within 20sec. Void and tube in vid both 5/8 in.

As described. Easy to mix, drys quickly.

I underestimated how much I would need will have to get more easy to use

This says 45 sec working time it very well means it. Mix fast and get it were you need it. Gets very hot in its reaction.