"Some people are under the impression that you can't obtain insurance while the car's in progress," If it's not registered, you can purchase comprehensive for whatever value it has while it's under construction. Then when it's registered and ready for the road, you can add liability and collision."
Many companies provide similar coverage for projects and we encouraged car owners to remember to upgrade their insurance as improvements are made during the build. After it's finished, continue to re-assess its value as more upgrades are made.
If the car is going back into a shop for an extended period of time, you may want to consider dropping liability and collision and keeping comprehensive until it is back on the road, when you can re-add that coverage.