Protecting the underside and chassis of a vehicle is an important consideration as often it involves a small initial expense and some time to do it correctly, but, in the longer term it can save massive amounts of money and lots of hassle as well as prolonging the life of your chassis and its strength. Most people can undertake such works themselves, so it is another periodic job, and most people will have the equipment, or able to improvise equipment to do the job to the standards of professionals.
Rust forms for one reason and one reason only, it is simply metals returning themselves to their natural state, their natural state is iron ore, and it occurs where any ferrous metal is subject to any electricity passing through it and has water and air in contact with it. Any metal structure contains electricity and this is not necessarily because it has anything electrical connected to it, all materials contain an element of electrostatic charge, so this cannot be eliminated, being i contact with air or water can. Air is all around us so we cannot eliminate these surroundings, water is also all around us, damp air, rain, and spray means we cannot eliminate this; what we can do is prevent moisture and air from coming into contact with metal and prevent metal rusting.
Manufacturers of vehicles have improved their rustproofing significantly in recent years, unfortunately they use what is called a "Performance Average System" for rustproofing their vehicles, so what is a performance average system? basically its one system which is adopted by a manufacturer for use across an entire vehicle range. Most of their range may be family saloons, small family cars, or people carriers as these are the most sought after vehicles and the most profitable for them to produce, and a performance average system works extremely well for the majority of their vehicle range. Such vehicles get significant protection for many years, but the system fails to be so effective on 4X4 vehicles for a variety of reasons, the predominant reasons being 4X4's having more open treaded tyres which throw more, and larger debris at a vehicle, are subjected to more underbody scraping, and often work in more contaminated conditions than the average saloon car. True 4X4's can accumulate more dirt in body cavities in 10 days than an average saloon can in 10 years, therefore the correct protection for a 4X4 is essential and beneficial in the long term.
Before deciding to perform any underbody protection, we need a basic understanding of what systems are available, and these are basically or loosely divided into three categories, these are:
Solid systems - solid systems are paints or metal coating systems such as galvanising, these adhere to a metal surface to enclose it from contamination, but lack any reasonable form of protection from continual blasting from debris thrown up from a vehicles wheels, and contact from scraping. Modern paints offer superb coverage, but need regular repairs to any scratching or scraping.
Flexible systems - flexible systems are the rubberised or flexible systems which are applied to vehicles, their coverage and protection is not as good as painted material but, being flexible means they are far more able to withstand constant poundings from blasting by debris and their flexibility simply absorbs most of this pounding without damage. Flexible systems rely upon being applied much thicker than paint, the thicker they are applied the more protection they offer, but they have an upper limit of thickness, if this is exceeded the material forms clumps and simply cracks with age and lets air and moisture penetrate it, and rust forms.
Viscous systems - viscous systems are basically thixotropic liquids which are applied in liquid form and remain in a liquid form, often they can be thinned before application, but thicken up as the solvent or thinning agent evaporates, these can be applied over traditional coatings, but are much better in concealed or inaccessible areas such as inside body cavities such as sills, inside chassis sections, and any enclosed area which cannot be painted.
Which system do we need, and which is best for our 4X4??????? an interesting question as the answer is all of them, we need the protection of a painted surface to eliminate air and moisture contacting the steel, we need inpact protection due to our open treaded tyres, and we need something which can be used inside enclosed and inaccessible body or chassis sections. Why do we need a combination of these systems? an interesting question.
When metals rust it forms ar rust on the surface of the metal, this spreads along the exposed surface and as the rust forms it eats away at the thickness of the metal, if we leave the rust it forms one long mass across and the thickness of the good metal is reduced, so we also lose the strength, this strength is important on any 4X4, particularly when heavily laden. Imagine the scenario, we convert a 4X4 to a camping or expedition vehicle so it is much heavier than the normal variant, this strength, particularly in a chassis, becomes much more important as any weakened chassis means more flex and more, or higher loads imposed on other structural components such as crossmembers or mounting points for other equipment such as fuel or water tanks. Higher than designed loadings on such components, and more flexing means the vehicle is more prone to cracked welds, not something we want if we are full of fuel and water and the tanks support brackets decide to snap when we are in a desolate or remote location, neither do we want a crossmember to crack as the chassis structural integrity is dangerously impaired. Imagine a chassis cracking near or at a suspension mounting point, particularly at higher road speeds.