By far the most common shells are made of acrylic with layers of fiberglass applied over it. The rough fiberglass provides the structural integrity, and the smooth acrylic skin is what gives it the colour and texture.
Some fiberglass shells are better crafted, thicker and much stronger than others.
But pay attention to the acrylic part of the shell. There are different quality levels of acrylic sheets. Some have antibacterial properties and wear better than others. Some are less porous, thicker and stronger than the cheaper alternatives.
Cheaper acrylics are more prone to fading, crazing, delaminating, cracking, blistering and splitting as it ages. Some are more easily damaged by UV rays and chemicals, and are more prone to “waterline etching”.
And it’s not just the quality of the acrylic that matters – it’s also how precisely and evenly the mold is pulled, how well the acrylic bonds to the fiberglass, how carefully the holes are bored, and how cleanly the sill perimeter is cut.
Poorly designed hot tubs and/or sloppy manufacturing techniques can result in the acrylic being over-stretched in certain areas, especially in the deep seats and floor corners. Sometimes the film is literally paper-thin and delicate. If water and/or air get between the fiberglass and the acrylic skin, it can begin to blister or delaminate.
Rotomold hot tubs are made of a polymer (google it so I don’t have to compose an essay on the topic). They’re cheaper to make but they are easily damaged by UV rays and extreme weather. They eventually start to crack as the oils in the polimer begin to deplete, causing the shell to shrink and pull.
Some brands can be fixed by plastic welding, but others use polymers that cannot be welded once the rotomold shell cures.
Some hot tub shells are made of plastic. Some claim that new technology makes them a viable option, but our experience so far has been that plastic is a terrible choice.