Piper Boat Owners' Club

A club open to anyone who owns a Piper-built boat



In the last article , I considered an overview of boatpainting. For this article, consideration is being given to the preparation of the bare steel and undercoating prior to applying the topcoat.


Firstly, in an ideal world, it would be sensible to use a complete paint system. Some are available, but they tend to be very expensive but one will be mentioned later.


When a narrowboat was built in the past, it was quite common not to remove the millscale, but to allow the external cabin sides to rust for a period of about 12 months. This allowed the millscale to rust off. The rust was then removed with a wire brush and the cabin painted with primer / undercoat / topcoat. Before fitting-out, the hull was coated with Bitumen, inside and out, later as materials improved, to be replaced with Comastic externally. It was vital that all the bitumen was removed as the comastic would not adhere to it. Internally, the cabin was coated in bitumen.


Today, with a new boat, the steel should be gritblasted before being folded and welded. Once it is welded up, the hull and cabin should be re-gritblasted and immediately etch primed both inside and out. The inside should then be spray foamed for insulation and the inside of the bottom of the hull coated with bitumen, whilst the outside is coated in Comastic. A variation on this theme is to have the steel sheets delivered sandblasted, etch-primed and folded. They are then welded after the etch-primer has been removed where the welding is to be done. Further primer is applied after careful cleaning.


Recently, there has been an article written about cold galvanising the outside of the hull, but this does mean that it will not be possible to coat the hull with bitumen or Comastic. The product in question is Zinga. Information on this product is available from www.zinga-uk.com - a division of Prosper Engineering Ltd. In my younger days, Galvafroid was used as the cold galvanising material, but it was very expensive. It is still available today and two coats would need to be applied to rust free clean steel. It is not recommended that it be overpainted, but in marine conditions it can be overpainted with a non saponifiable finish paint – clorinated rubber paint. Galvafroid is the trademark of Fosroc International Ltd. Cost is approximately £25.00 + VAT for 800ml. It is expensive! Information is available from www.fosrocuk.com . I think that these two products are for information only!


Davids Zinc 182 anti-rust primer is another good zinc rich paint, which in theory does not require an undercoat – 2 coats of primer is all that is recommended. The general view is that a couple of coats of undercoat is a good idea – this should be matched to the final topcoat.


Various other materials are now used in the construction of narrowboats. My cratchplank is made of aluminium – a lorry floorboard which is hollow, but welded all round so that it will float if it falls into the water! It is important that aluminium is insulated from steel otherwise a galvanic action occurs and there will be corrosion seen in the aluminium with holes appearing. Out of interest, many of the earlier Land-Rover Discovery’s had this problem, holes appearing in wings and doors as they were in contact with steel framework and had not been properly insulated.


Heavy plastic hatch doors are also becoming more popu;ar – wood is no longer suitable, nor indeed desirable. Endgrain plywood even if adequately sealed, will rot quickly – marine grade is no exception. Both plastic and aluminium require etch priming with appropriate primer for the given material, they are then treated as normal. Hardwood is very difficult to paint, hence the reason for treating it with Danish Oil or a marine varnish, the latter rapidly coming off. If hardwood requires painting, then it should be left to weather for up to 6 months, allowed to dry and then given a couple of coats of raw linseed oil, which is left to harden for about 4 weeks, before priming and treating in the usual way.


Galvanised steel is not suitable for welding. There is, however, a product known as Zintec manufactured by Corus. This material has a zinc coating and can be welded without any problem. It is then rubbed down and etch primed as for zinc. It is then painted as normal. It is only available in up to 2mm in thickness, but would be suitable for making hatch doors, folding the steel around a plastic sheet and welding on the hinges.


Preparing the boat for painting – if most of the paint is sound, then it may be left and given a thorough sanding – see below. Areas of rust should be wirebrushed using a grinder and then sanded. Rust around windows is best managed by removing the window and treating the rust with a wire brush. If, however, the paint system is a bit shot, then consideration should be given to having the boat grit-blasted by a professional. Immediately the bare metal has been sanded or grit-blasted, it should be primed. All bare metal should be primed, but areas of paint that have been left should be undercoated after sanding. It goes without saying, that bare metal should be dry sanded!


Once the primer has been applied and rubbed down using wet and dry abrasive paper 250/500 grade and any imperfections filled with cellulose putty, the business of applying the undercoat must be considered.


Paint may be applied by brush, roller, then brushed out to remove the air bubbles or by paint spraying, the last requires skill with a spray-gun and is better left to the professionals, in general, boat painters are not in this league!


Rubbing down is best accomplished using a palm sander that is air driven – not a good idea to mix electricity and water in wet sanding!


Finally, it is difficult to get a good join between primed metal and undercoat – there is a great deal to be said for starting from scratch.


In the final article, I will talk about paint systems amd re-sealing windows that have been removed to paint the boat.

Dugald Campbell

Return to Pipeline Index