Sealcoating is a wonderful way to protect your investment in your agricultural equipment. It isn't a magic solution, though. You need to know some basic things before you break out the sealcoating tools and get to work. 

Purpose 

Sealcoating preserves whatever the existing condition of the surface is. Suppose you have a tractor that has an old paint job that's beginning to chip. Rather than go to bother of repainting the vehicle and then still needing to put a protective coat on the surface, you can simply seal coat the cracked paint. The product will fill in the gaps between the blocks of good paint, preventing water and pollutants from getting into the gaps and causing deterioration.

Rust

Typical sealcoating products have no penetrating value so you need to be especially careful about using them on rusty equipment. Ideally, you should be able to use a wire brush to clean the surface and get a clean area for treatment. Lots of equipment is going to be rusty enough that you'll have to use a power sander to break through the rust.

Some extreme cases may require you to apply a rust inhibitor. This doesn't stop the rust, but it does slow its progression. If you sand as much as possible, pressure wash, use an inhibitor, paint the surface, and apply a sealcoat, then you should be able to extend the life of the materials for at least a few years.

Make sure all of the debris and deteriorated metal is gone before you apply a sealcoating material, though. You don't want to seal anything bad against the metal because it can act as a node for creating more rust.

Pooling Water

Plenty of equipment also sits in locations where water will pool on it. For example, the bed of a trailer might sit outside of a barn in the rain and snow. Using sealcoating equipment and techniques, you should be able to protect the equipment from rapid deterioration under the weather.

Notably, you should do this preemptively to achieve the best possible results. Many people get out their sealcoating tools the minute they buy something. Adding a layer of protection won't hurt on most new paint jobs.

Rough Surfaces

Especially in an agricultural environment, paint often wears unevenly due to dust and small rocks hitting machinery. If the painted surface feels rough to the touch, that's a sign that it might be wearing down. Even if there's no visible chipping or rust, you may want to sealcoat the surface.