When your craft or boat blisters in the sun

**When your craft or boat blisters in the mid summer heat**
or
Heat Deflection Temperature / Heat distortion Temperature ( HDT )
Glass Transition Temperature ( Tg)
Guess you will ask me now what I'm talking about - okay lets start with what we see and go from there. You have your craft finally finished glassed over the foam core panels and painted - several layer of paint over the fairing compound produced a mirror like finish and you are fully satisfied with the result. Time to crank the lift engine and let her lift off - look at here and enjoy - just the muffler is blowing with full power against your great paint job - once the engine is shut down you take a rag and clean the now black messy spot so your pride is again visible in the shiny paint job - bummer - small black marks are now visible and you can see the fiberglass fabric's woven structure through ... What happened ? This is a very rude and quick visible aspect of HDT - and Tg. Another example is that you painted your craft in dark colors and while it was sitting on your trailer in the mid summer Texas heat - you cooled of with a few cold drinks - she blisters like a BBQ chicken on the rotisserie grill. - Sorry the same story as above in just dark colors. So back to HDT : Heat deflection temperature is the point ( or temperature) where your cured cross linked resin will change its physical properties. Tg : has several definitions : A wide range of materials exhibit glass transition temperatures, such as the temperature at which a material changes from being rigid and brittle to becoming rubbery and flexible with a considerable reduction in mechanical properties. The temperature at which increased molecular mobility results in significant changes in the properties of a cured resin system.