F21 Wood Block Adhesive. STYCCOBOND F21 is a high temperature grade bitumen/rubber emulsion adhesive designed for application by dipping. It is non-flammable, protected against bio-degradation and is suitable for use over normal underfloor heating installations. STYCCOBOND F21 can be used for securing individual wood blocks at least 12mm thick, made from a wide variety of woods. STYCCOBOND F21 can be applied to STOPGAP smoothing underlayments, structurally sound, smooth, dry, absorbent subfloors of concrete and sand/cement screed and plywood (minimum 6mm flooring grade).