A specially formulated wax which penetrates rust and dirt preventing further corrosion. Use on open surfaces or cavities. Even damp and dirty surfaces will not affect its superb performance.
To clean use white spirit or solvent and elbow grease. Not recommended to thin it down with anything.
Steam clean the underneath of the vehicle and allow to dry, remove any flaking rust and mask up any areas where you do not want the wax to go, time consuming but well worth the effort as it is difficult to remove any unwanted overspray later.
Treat any surface rust with the rust converter (3125HS), once it has turned black proceed to treat the cavities with the cavity wax also giving a fine coating of cavity wax to the underbody of the vehicle with any you have left over as it contains rust inhibitors.
Leave overnight to dry and then apply the underbody coating (4941), this will dry to a leave a finger print in it type finish as it remains soft underneath so it can migrate and reseal itself when it gets a stone chip. When dry remove the masking and away you go.
Don’t forget to wear something you can throw away afterwards.