Background Reading: 6-F: Shell Condition

As explained elsewhere in this website, a shell in good condition means very little.  But a shell with damage is a big red flag.  Small spiderwebbing and tiny cracks will not likely cause the tub to leak because the acrylic is only a thin membrane stretched over a more robust fiberglass shell, but it may affect the aesthetics.  All-out cracks and blisters are an indication that the shell is beginning to fail, and will only get worse unless professionally dealt with on time (often an impossible or cost-prohibitive repair).

Tips:

-Check along the waterline for chemical etching.  This white line is shell damage, and cannot be removed by scrubbing.
-Metallic and pearlescent colours are more prone to crazing (cosmetic only), and the top sill part of the hot tub is where this sort of damage is most evident. Shift your head back and forth using the sun or a flashlight to cause a reflection to check for spiderwebbing, crazing and small fissures.  These cannot be buffed out.
-Check under the shell sill (where you can see where the acrylic and fiberglass are bonded) to look for signs of delamination.
-Carefully inspect the deepest concave corners and the floor perimeter for thinned-out or cracked acrylic or blisters.  
-Check for all-out cracks (longer lines, sometimes with raise edges) These are likely to spread and grow as the tub ages
-Check to see if any dry areas are lighter than the wet areas.  Faded shells don’t look faded when they’re wet.
-It’s sometimes hard to notice shell damage when it’s full of water, so look very carefully, don’t just do a quick glance.