Sanding and Warranty
Haven't heard anything about sanding the gelcoat on a 361 in particular causing the warranty to be voided but I've heard statements to this effect for other boats, especially with regard to blister warranties. The best source for a definitive answer with regard to this question would be the factory or boat manufacturer.If one is contemplating bottom work and you want someone to stand behind the work when it's done, it's certainly a good idea to get all your ducks in a row. That is, try to get what is required from boat manufacturer, the barrier coat/bottom paint manufacturer, and the yard. Getting the materials manufacturers rep to make an inspection when the work is in progress is also helpful. Take pictures of the work and make note of whether conditions and keep track of work progress and who is doing what when. Ensuring the yard understands the manufacturers requirements and they will comply with them is key to a good comfort level. Sometimes a yard will "bust the limits" of temperature, humidity, or whatever, even after having an agreement or understanding, because "they've done this before" so be careful. Some yard people will give very convincing arguements.Get manufacturers technical literature for the products to be used. This info is much more detailed than the consumer stuff at the chandlery. Try not to (i.e., dont') mix manufacturers products on the same project. Keep a record of all work and chemical products used as a part of the boat historical records. Later, 5 or 10 years, you might be back to that area of the hull doing more work of some kind and having the history of what happened will be useful.