True it is a good product. However, if the 5-10 year old frame is already rotted on the inside, nothing can help it! If you replace the tongue, then the internal frame coating would be a great option, as well as many other products. Just got to get something on it while it is solid! If you can remove the actuator and inspect the inside, maybe borrow a boroscope, to ensure the tongue is in need of replacement, or solid enough for coating.
Look at my original post, the tongue LOOKED fine, until i touched it..and it fell apart.
When we launch our M, the tongue almost always gets submerged, and the hold where the brake line exits is too high and holds in the water (on the all steel trailers anyway) A drain needs to be drilled in the bottom rear of the tongue to drain the water.
If the tongue is even questionable, buy a new section of 3x3 and coat the inside and weld in place with a new outer member, and likely , new actuator