Applying finishes to a cleaned underside is within the capability of the average 4X4 enthusiast, but it is better if you have a compressor or access to one and the various tools to apply some of these coatings, but not totally necessary as there are alternatives if people apply a little lateral thinking and ingenuity.
Once an external area has been cleaned it needs its first layer of protection, this would normally be a layer of good quality industrial metal paint, or an easily obtained quality metal paint such as Hammerite or similar, a good coat should be applied, or two coats if finances permit. Care should be taken to apply a good coating around rust prone areas such as chassis welds as these often form little voids in the metal and rust continues to form.
Once the paint has been applied it needs time to go off, now we have our solid coating to exclude air and moisture, and the first stage of our protection.
Applying our second layer becomes a little more difficult as we need to apply a flexible coating, this is commonly called Stonechip and comes in bottles which screw to a dedicated compressed air powered applicator gun, if you have a compressor its beneficial to obtain one of the applicator guns as they cost less then £15. If you do not have access to a compressor than you can get spray on variants of stonechip, but these cost considerably more per container, and contain much less material, and they work at lower application pressures than compressed air powered applicator guns, and sometimes spraying into recessed areas can be difficult. Stonechip normally comes in two colours, the traditional colour was grey, but now white is available, and it can be oversprayed with primer, paint, and other underbody protection materials.
With a compressed air powered applicator gun you can dictate the finish of stonechip by varying the air pressure at the gun, so replicate the original manufacturers finish, never spray stonechip over rusty metal at rust continues to form underneath the coating. Apply one coat and allow it to go off for at least 1 hour on a hot summers day, or overnight in winter, and apply at least one more coating, two more coats would be better, now we have a solid and flexible protection.
If you have a stonechip gun, and stonechipped the underside of your vehicle, save the pots your stonechip came in and clean them, you can use these for applying underseal with your stonechip gun, and without paying the high prices for underseal which comes ready for use in containers similar to stonechip containers.
Our final outer coating is underseal, you can buy this in large tins or ready for use in stonechip containers, a 1 litre container ready for use on a stonechip gun is currently around £9, a large 5 litre tin of underseal is currently around £12 per tin, white spirit is around £2.50 for a 2 litre bottle. Therefore you can buy a 5 litre tin of underseal and 2 litre bottle of white spirits for less than 2 litres of underseal ready for use on your stonechip gun.
Using an old mixing jug, half fill with underseal from your tin, add white spirits and mix until it is about the same consistency as your stonechip was, pour into your cleaned and saved stonechip containers ans screw to your applicator gun, apply to the underside of your vehicle and you will have a mottled finish exactly the same as the manufacturer applied finish. Apply several coats to the entire underside, and allow time for them to harden between each coat, apply more coats to more vulnerable areas such as the underside of wheelarches or other vulnerable areas, you will now have a protection system on your vehicles underside which will last for years with little more than a periodic touching up.
Protecting the inside of your enclosed chassis or other enclosed sections is a little more difficult, with the outside of the chassis and floor completed you need to ensure the inside is just as good as most chassis or other enclosed areas tend to rust from the inside out, and you cannot often see inside a box, or other enclosed section. Choosing your preferred internal rustproofing system is crucial as care and consideration needs to be given to which system you choose, the two main systems are Waxoyl and Dinitrol and both have pro's and con's.
Consider how you intend to apply your rustproofing system, Waxoyl to a cheap extended hand pumping lance for getting into box sections which is fairly cheap to purchase, Dinitrol is more industrially based, so uses special compressed air powered applicator tools which are considerably more expensive and require a compressor. Also consider the solvent which thins each system as these differ between products.
Once you have made your choice its time to buy your equipment, you should apply the coating using the applicator guns, i prefer to begin in the centre of a chassis and work outwards, always aim towards the top of the chassis as gravity will let the rustproofing liquid run down the sided if the internal box section. Always apply plenty of rustproofer so the entire area is covered, and plenty runs down the sides of the chassis and along the bottom, this means you will invariably get lots of rustproofer dripping out of any bottom holes in the chassis, crossmembers, or sills, so have plenty of tins on hand to place under any drain holes to catch the surplus. Any surplus which is caught can be filtered through a pair of the wives/girlfriends tights for reuse. If you get any rustproofer on the floor then it will be difficult and messy to remove.
Once you have done the centre of the chassis you can work towards each end of the vehicle using any holes in the chassis, once you are near the end you can gain access through each end of the chassis to ensure its fully coated, many people prefer to repeat the procedure a second time to ensure full coverage of the entire inside of the chassis/crossmembers.
Once the chassis is fully coated its time to move to the enclosed sections such as sills, the same procedure is followed as that of the chassis, fire the rustproofer at the top of the sections and let it run down, and just appy plenty of it, and place tins under the drain holes to catch any surplus, filter, then reuse.
If you cannot afford the spraying equipment you can thin down the products with the correct thinning solvent and use an old garden sprayer, fill with the product, pump up the sprayer and apply, most have an adjustable nozzle ranging from a jet to a mist, and an angled head, so just set the nozzle and spray.
One other option exists, if you cannot afford the cost of proprietry products then use old engine oil, just thin it down and spray this in in exactly the same manner as the rustproofer.
Internal rustproofing needs periodic topping up, depending on vehicle use this is around every 2-3 years.