Aliphatic unsaturated polyester resins providing excellent weathering resistance. Use of a gelcoat providing excellent weathering resistance, UV resistance, reduction of yellowing and retention of gloss that is especially suitable as gelcoat for exterior glass fiber surfaces such as sailboats, motorboats, yachts and mobile homes. The gel comprises an aliphatic unsaturated polyester resin characterized in having a viscosity of 100-1000 mPa s (25° C.), a monomer content of 40±15% and an acid number of max 15 mg KOH/g.