While the adhesive classes in this specification are intended for general-purpose applications, the user should satisfy himself with their suitability by preparing and testing specimens with the intended adherends, taking care that the surfaces of such adherends are properly prepared prior to bonding. Surface preparation methods are detailed in the technical literature and can often be obtained as recommendations by adhesive manufacturers for their particular formulation. The adhesives, being thermoplastic, are not recommended for applications where they will be subject to either continuous high stress, or stress at an elevated temperature. The class 2 adhesive (see 1.2) with its higher solids content forms heavier films with controlled penetration and is used for porous surfaces.