Which process should be repeated several times when using varnish?

The process of applying varnish and sanding is critical in achieving a polished and smooth finish on a surface. When varnish is applied, it typically does not dry to a perfectly smooth surface on the first coat. Instead, it may have imperfections such as dust particles, bubbles, or a rough texture.

Repeating the applying and sanding process allows for each layer of varnish to build up gradually, while the sanding helps to remove any imperfections from the previous layer. This method not only enhances the glossy finish but also increases the durability of the final work by ensuring that each layer adheres well to the previous one. Consequently, taking the time to apply varnish in multiple thin layers and sanding between each application results in a higher quality finish that is both aesthetically pleasing and functional.

The other options do not directly relate to the finishing process of varnishing. Applying a base layer of oil may be part of a different preparation process, sketching the design is relevant to the planning phase, and cleaning the brushes is an important maintenance step, but it does not involve the actual application of varnish and the refinement of the surface finish.