An electronic device may have structures such as housing structures, display structures and other device structures that form gaps when assembled. To prevent intrusion of moisture and other environmental contaminants, the gaps may be filled using gap sealing material. The gap sealing material may be a liquid polymer that is applied to the gap in a way that creates an excess protruding portion. Light-based processing techniques and application of solvent may be used to remove excess material. A sealing material such as an adhesive may be applied to gaps to forming a sealing structure. A material may be applied to the adhesive sealing material to cause the sealing material to expand and fill the gap. An undersized gasket may be placed in a gap. The gasket may include materials such as polymer that expands upon application of moisture.