How thick should it be for a rust proofer?
Anywhere between 15 mils and 60 mils would be suitable depending on whether or not you want to provide full undercoating application for rough service or sound deadening. Many of our products are 3 part products and include a powdered component to allow for variable consistency depending on the application. For just rust proofing, a thinner coating of just the liquid parts would normally be recommended and probably be about 15-20 mils. Actual undercoating would probably consist of about 50% powdered product additive to allow you to quickly build a thick coating.