For sure do not use electric or steam tracing on the outside of a FRP to get the contents to 90oC.
Steam (low pressure, 15 psi) HXers & electric heaters are used all the time to heat metal finishing solutions in open top tanks. Electroless nickel (EN) typically is plated at 90oC in polypropylene tanks. Of course, plenty of support is used -- full-bottom, girth ribs & top flange.
If using an electric heater in an EN solution, be sure it is 'de-rated' [more surface per wattage, so don't locally overheat the solution]. Also, use support legs & standoffs for better circulation & to avoid contact with the plastic. Plus, use liquid level-controlled safety shutoffs on the heater -- electric heaters are the no. 1 cause of fires in plating facilities.
What are you heating?