Another factor that cannot be ignored is the form of the adhesive. Solvent-based adhesives tend to cause wrinkling at the joint edges when bonding thermoplastic films or their composite films. When used on rigid thermoplastic materials, however, they often reduce the need for surface treatment. The special shape of the adherend usually favors the application of specific types of adhesives. Bonding aluminum honeycomb structures to flat metal sheets is best achieved with a liquid primer and thermosetting adhesive film (backed with fiberglass cloth). Paste adhesives allow for convenient assembly of heat exchangers from copper fins and aluminum tubes. For porous materials, high-viscosity or cream adhesives are more suitable.