3M Short Strand Reinforced Filler is reinforced with short strand fiberglass to create water resistant repairs that are twice the strength of ordinary body fillers. It is a great solution for repairing holes and rust on metal and fiberglass.Thicker and More Malleable3M Short Strand Reinforced Filler is thicker than regular filler, so it requires a bit more effort to mix. Compared to long strand fiberglass filler, short strand filler is also a bit more malleable and softer. Fiberglass filler is great for applying over welded areas and can be used as a build layer where thicker applications of body filler are needed. A Unique Line of Reinforced Filler Products from 3MAltogether, the 3M line of reinforced filler products offer users a unique and flexible array of repair solutions. Each product is reinforced with short fiberglass strands, long fiberglass strands or milled fiberglass. These reinforced fillers are significantly stronger than regular fillers and are waterproof. They are great for repairing fiberglass and rust holes in metal.