That's the issue, hardness. Hardness translates into a tendency to crack and break. Epoxy is a very, very good material, properly used. Thin coatings, unreinforced, are not a recomended use.
Epoxy is, in this sort of job, properly used as one component in a composite material. Composite materials of this type deploy the complementary strengths of multiple materials to create a new material better than it's parts.
The whole reason epoxy is reinforced is to take care of the hardness / brittleness. and give the composite material the monolithic nature and compressive strength of the epoxy, and the ability to take impact and the tensile strength added by the glass fibers.
If epoxy was tough enough on it's own, reinforcement would not be common. The same logic results in the reinforcement of concrete. Concrete is a great material, and it has great compressive strength, but it does poorly under tensile loading and under impact. Steel mesh within a concrete plate gives it the strengths it lacks on it's own.
A sink will be impacted by every item that gets put down on it, and any item too sharp, or placed too heavily, will fracture the epoxy coating and let liquids get trapped into the MDF, which has very minimal resistance to degradation from water. It just takes one small crack. Once a little water gets in, it swells the MDF, which will further crack the expoxy coating. It becomes a self degrading system. An unreinforced epoxy coating is a much less than ideal companion for MDF in a wet environment. MDF needs full time babysitting.
It might work, but I wouldn't do it that way myself or suggest anyone else try it, without at least trying to communicate the likely mode of failure. The glass fiber is far cheaper insurance than ripping out a sink to re-do it. I think it's false economy.
C
