Neat epoxy resin is a poor gap filler on its own — it's designed to bond and saturate, not to bulk out a void or hold its shape in a fillet. That's what fillers are for, and we stock three that get reached for constantly but do genuinely different jobs. Picking the wrong one doesn't usually mean outright failure, but it does mean more sanding, a weaker joint, or a finish that's harder to get smooth than it needed to be.
MAS Epoxy T-Cell: Maximum Strength, Minimum Sandability
MAS Epoxy T-Cell is a cellulose fibre filler built for jobs that need extremely high strength and resistance to cracking — structural filleting, gluing, bonding hardware, and keel bedding. It makes an excellent thick paste for bedding bulkheads into a hull, where you genuinely need the joint to hold under real structural load. The trade-off is that T-Cell is extremely hard to sand, so it's the wrong choice anywhere you need a smooth, finished surface. The standard approach is to apply an over-fillet of Workshop Mix or Workshop Mix 10/20 on top once the T-Cell has done its structural job, since that sands far more easily to a clean finish.
MAS Epoxy Colloidal Silica Filler: Hard, Dense, and Good for Fairing Too
MAS Epoxy Colloidal Silica Filler forms an extremely hard, high-density solid when mixed into resin, which makes it well suited to structural bonding, filling and filleting work in its own right. It also has a second life as a fairing agent, where it works well for hiding and blending repairs on GRP boats. If you're bonding, filling or filleting with Colloidal Silica, MAS recommends using it with Medium hardener alongside FLAG resin specifically.
MAS Epoxy Workshop Mix: The General-Purpose Bulking Agent
MAS Epoxy Workshop Mix is our own blend of silica, cellulose fibres and wood flour at a set ratio, and it's the main filler we use in our own workshop for general gluing, fairing and filleting where T-Cell's maximum strength isn't the deciding factor. It builds a peanut-butter to ketchup consistency depending on how much you add, works well as a filleting compound around bulkheads, and holds up as a fairing compound in areas that will see harsh wear. One trick worth knowing: when laminating hulls, forcing Workshop Mix through pre-drilled holes in each ply layer stops air pockets forming between layers, and once sanded back, the cured plugs act almost like epoxy nails, adding real strength to the bond between sheets.
Making the Call
As a rough rule: reach for T-Cell when the joint has to hold structurally and you're prepared to finish it with something else on top; reach for Colloidal Silica when you want that same hardness but with a second use as a fairing agent; and reach for Workshop Mix for everyday gluing, filleting and fairing where you don't need the absolute maximum strength of T-Cell. If you're still unsure which filler suits a specific repair, our contact page is the fastest way to get a straight answer before you commit resin to the job.
