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Is sand blasting necessary

When having a boat repainted is it necessary to remove the metal oxide by sand blasting or do modern primers key and prevent micro-blistering.I have heard International Paints only guarantee their paint if the surface has been sand blasted.

Asked by: grayham pope  | 2.29pm, Tuesday 28 May

WW says:

Grit blasting (to standards SA2.5) is really the only way to completely remove oxidation and millscale (the initial layer of surface oxidation that occurs when steel is rolled). The hull is usually the most at risk of corrosion and grit blasting here certainly is effective. Indeed, if you wish to use two-pack epoxy finishes like International's Intertuf, it is really the only option. Also, these types of paints will not key themselves effectively to older coatings.
On the cabin top, if the corrosion is limited, then wire brushing, or using a rotating disk etcher, like a Tercoo unit, can bring the surface very close to that of grit blasting. It is also possible to remove the surface corrosion and then treat with compounds such as Fertan, which convert any remaining oxides to an insoluble compound which provides a good base for paint. Indeed, I have used this quite successfully on a cabin roof that was heavily pitted and corroded. Filler was then applied after the first two coats of primer, before recommencing the final repaint.
Interestingly, paint manufactuers have had to reformulate paint over the last few years, to reduce levels of some components, particularly volatile organics. This means that good preparation of the surface, including thorough degreasing and drying, is important, as well as dry application (and letting the paint cure sufficiently between coats) to avoid microblistering. Leaving a boat in primer or undercoat wet for even short lengths of time can have apparently detrimental effects on the final coats.
I hope that helps- please do ask if we can be more help.

Mark Langley  | 9.01AM, Wednesday 29 May

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