A steel boat owner’s biggest enemy is corrosion. They don’t have to worry about osmosis or rotting timbers, instead rust is the number one issue that will keep them awake at night.
Steel hull maintenance mainly involves checking inside and outside the hull for rust and then dealing with it. This means sanding, chipping, scraping, grinding or blasting rust out until the metal beneath is shining. The metal then needs to be covered up as soon as possible with multiple layers of rust converter, primer and paint.
It is just as important to deal with any corrosion that may be present on the inside of the hull as well as the outside. It is a widely acknowledged fact that steel hulls generally decay from the inside out, so it requires dedication from both the builder and owner of a steel boat to ensure that every nook and cranny inside the hull is cared for almost to the point of fanaticism.
It is worth noting that with modern painting systems, if a well built steel hull is backed up by being properly prepped and painted at the build stage, then it need not be too arduous to maintain. Rigorous attention does need to be given to prevent rust from forming. The time spent doing regular checks and dealing with rust as soon as it appears will reap benefits in the long term.
How to repair rust spots
- Begin by chipping out the rust using a chipping hammer or a grinder with a very coarse16 grit grinding wheel. Remove any loose paint with a putty knife.
- Use a wire brush to clean the surface thoroughly.
- Sand and feather the surrounding paint surface so that it has a smooth edge.
- Clean and degrease the surface before applying a metal primer.
- Apply the epoxy primer and leave it to dry.
- If there are any deep spots these can be filled using an epoxy filler. Apply the filler with a putty knife, smooth it back and leave it to cure.
- Sand the surface smooth using a 200 grit paper. Wipe clean with acetone and apply another coat of primer. Leave to cure and it will then be ready for the topcoats to be applied.
Stripping back larger areas
For larger areas, there are a number of options. Check with your boatyard which of these options they will allow you to use. Note for all these options the use of full protective clothing and eye protection is recommended as they all generate a lot of dust and dirt:
- Needle guns – these tools work use compressed air and are very noisy. They have vibrating needles which chip away at the rust and paint and are particularly good for getting into tight corners inside the hull. Good quality ear defenders and eye protection are essential.
- Shot blasting – there are several types of shot blasters, including dry and wet variations, some which use sand or grit, others which use soda. They all require a compressor to work. As well as getting rid of the rust, shot blasting prepares the surface very well for painting as it scratches the surface, which helps paint to adhere to it.
- Rotary blaster – these come in various forms from drill-mountable rotating discs to dedicated machines complete with guard and dust extractors. They are all designed for the removal of rust, paint, tar and other materials, which leaves the equivalent of a sand blasted surface with a texture suitable for applying protective coatings. For DIY purposes, the rotary blaster is well worth considering as it can be used on most types of electric drill.
- Polycarbide abrasive discs – these are fitted to a standard angle grinder. They are effective at cutting through paint without damaging the steel. They work best over smooth areas but are not as good dealing with deeply pitted areas.
Rust converter and preventer
Apply a rust converter and preventer to the freshly blasted steel surface. Rust preventer is basically phosphoric acid and converts rust to iron phosphate. Leave the rust converter for an hour or two and then brush it over with a hard bristle brush to remove the residue left on the surface. The rust converter provides an insoluble, non-conductive and oxide-free surface over which paint can be applied. The surface will now be ready for priming and painting.
Fairing – sanding and filling
Boat hulls of all kinds will require filling and fairing from time to time. Fairing is a time consuming and arduous process but can substantially enhance a boat’s appearance, as well as providing additional protection for the hull. While the appearance above the waterline is one consideration, beneath the waterline the priority is to ensure that the hull is as smooth as possible in order to offer the least resistance through the water.
Fairing compounds use either polyester or epoxy based systems. In the case of polyester, using a small amount of catalyst results in a hard plastic finish. With epoxy, the resin reacts with a hardener and they form a chemical bond that also results in a hard plastic material. The big difference between the two is that epoxy systems are inherently more water resistant, shrink less when they are cured, are more adhesive and are stronger than polyester resin. Hence epoxy fillers are generally recommended for marine use.
Applying the filler
- On sand blasted metal, first wipe with solvent then apply a pre-coat with a thin solvent-free epoxy resin mix. Then apply the filler mix while the pre-coat is still tacky.
- On primed metal, the surface needs to be sanded and then wiped with quick evaporating solvent to ensure the epoxy filler has a good key.
- For small areas, apply the filler with a plastic spatula or a metal straight edge. Aim for a slight overfill so the repair can be sanded back flush with the surrounding surface.
- On larger areas, apply the filler using a flexible metal applicator that enables you to spread the filler over the surface.
- On larger areas, it usually requires two people to work together with a fairing batten, spreading the fairing compound evenly and smoothly over the surface. The aim is to apply no more than 3-4mm at a time and end up with a slight overfill that can be faired back when the the filler is cured.
Once the filler is fully cured, the objective is for the excess filler to be sanded back to leave an even, smooth surface. This usually needs to be done in a number of stages, beginning with coarser grit paper and finishing off with a finer 200 or 240 grit paper.
Getting this right in one go is not always possible. The solution is to apply fresh fairing compound over any remaining uneven or pitted areas. Once again, after the filler has cured then you can sand it back until you are satisfied with the finish.
And then, it will be time to begin the painting process.