Link: https://hackaday.com/2025/11/29/on-the-benefits-of-filling-3d-prints-with-spray-foam/

Closed-cell self-expanding foam (spray foam) is an amazing material that sees common use in construction. But one application that we hadn’t heard of before was using it to fill the internal voids of 3D printed objects. As argued by [Alex] in a half-baked-research YouTube video, this foam could be very helpful with making sure that printed boats keep floating and water stays out of sensitive electronic bits.

It’s pretty common knowledge by now that 3D printed objects from FDM printers aren’t really watertight. Due to the way that these printers work, there’s plenty of opportunity for small gaps and voids between layers to permit moisture to seep through. This is where the use of this self-expanding foam comes into play, as it’s guaranteed to be watertight. In addition, [Alex] also tests how this affects the strength of the print and using its insulating properties.

That's a good idea. I was just using spray cheese and fish kept sinking my creations.

I was under the impression self expanding foam isn't water tight, just air tight.