When does a shop protect parts from corrosion?

The correct answer highlights the importance of using a weld-through primer when replacing welded parts. This primer is specifically designed for areas that will be welded, allowing for adequate protection against corrosion while ensuring proper adhesion during the welding process. It serves a dual purpose: protecting the metal surface from moisture and corrosive elements while also maintaining the integrity of the weld joint, as traditional primers can burn off when exposed to high temperatures.

As for the other options, while they do address corrosion prevention in various ways, they may not be as effective in the context of welded parts. Applying paint after installation can provide some surface protection but is not always sufficient for areas exposed to high heat during the welding process. Using rust-resistant materials can enhance durability, yet the welding process can compromise the protective finish that these materials may have. Covering parts during storage can help prevent initial exposure to moisture but does not account for protection during the assembly or in-service phases. Hence, the application of a weld-through primer is a critical step specifically tailored for protecting welded joints against corrosion.