What method can enhance the retention of bis-acryl when used for veneer provisional?

Spot etching the enamel is an effective method for enhancing the retention of bis-acryl when used for veneer provisionals. This technique involves treating specific areas of the enamel surface to create a micro-roughened texture. The microscopic irregularities formed by etching increase the surface area available for mechanical retention, allowing the bis-acryl material to bond more effectively to the tooth structure.

When the enamel is spot etched, the bond strength between the provisional material and the tooth surface is improved, which is especially important in maintaining the provisional's position during its use. Bis-acryl materials rely on both mechanical retention and adhesive bond strength, and enhancing these factors through surface modification can lead to better clinical outcomes.

In the context of the other options, while ensuring preparation wraps around the lingual surface and including contacts may provide some degree of retention, they do not specifically address the bond strength enhancement that etching does. Creating undercuts in the dentin might compromise the integrity of the preparation and is not generally advisable for provisional retention; instead undercuts need careful consideration to avoid difficulties in removal.