GRP is lightweight, yet that also makes it a lot flimsier than metal.
GRP is subject to manual fabrication, while metal is mostly automated. GRP will be cheaper to produce for big quantities of identical pieces, while metal is more cost-effective for individualized planters or large structures.
Still today, GRP molds are generally created by hand. It is suitable for complex sculptural forms. Modern sheet metal technology is more precise, making metal more ideal for geometric forms and large projects, but less practical for complicated curves and sculpting.
The surface finish options for GRP are more limited – and any effects you can achieve on GRP can be finished on metal, too. In addition, metal is predisposed to smoother surfaces and sharper edges.
Not all GRP is created equal. The resins and fibers used + the quality and fabrication methods vary, making for erratic end products. Metal, conversely, is subject to universal classification that is incontestable.
It’s hard to prove endurance levels of GRP, because it varies based on the level of workmanship. On the other hand, there are reliable stats on how long you can expect metal to last, based on the aging characteristics and maintenance requirements. That makes for greater certainty and better warranty options for metal.
Even a small bump into GRP could cause it to crack or split, and the damage is usually irreversible. Metal can withstand strong collisions, save for minor dents that don’t affect the function of the planter. Such dents in certain surface finishes, such as corten steel, are barely discernible.
It is complicated to add seating, trellises or fencing to your GRP-designed planter. In all likelihood, metal parts would need to be incorporated internally to support the structure. Metal harmonizes aesthetics and function, with extra parts integrated in the design of the metal itself.