Thursday, May 17, 2012

Bottom paint

The below the water line part of the boat is painted with the barrier coat. The boat can now be removed from the building cradle.

Having purchased the metal for the boat wheel abraded and primed, getting her bottom ready for the barrier coat was not to bad of a job. There is not much good anyone can say about sand blasting, but as far as sand blasting a boat this size, this job could have been worse. While grinding metal may make the metal look like it is in an ideal surface for paint, the profile created by sand blasting is the gold standard for a mechanical bond of paint to metal. Because the metal had already been blasted, my focus on preparing the hull was to blast to white metal the welds and weld zone, all the print through from the interior frames and longitudinal stringers and the grinder marks. All the rest of the metal was blasted to "scuff" the existing primer to give the barrier coat paint a profile to stick to.

I had talked to a commercial blaster about doing this job, and while his price was reasonable, I felt as if I had  decent enough equipment to do the job. The air compressor in the barn is a single phase 10 hp with an 80 gallon tank. Because the mill scale had already been removed, and I was not taking all the hull to white metal, the barn air compressor could handle the job. Before starting blasting, I cleaned the shop and gave the floor a good sweeping with a back pack blower. The blasting pot I used only holds 100 lbs, so after the pot was emptied, the sand was swept up off of the shop floor, poured through a screen and re used. After three screenings, the sand was thrown away. The weather was about as perfect as one could fine to sand blast with 70 degree temperatures, high pressure, and darn near zero humidity. Blasting was started late in the afternoon and finished the next morning. About eight hours was all it took to do the job.

Because the blasting was done inside during picture perfect weather, blooming of rust was not an issue. Before starting the first coat of paint, the floor was swept again and the hull blown off with compressed air. The paint I decided to use is a PPG product called Amerlock and is a two part paint. My paint rep has been selling it to the operators of push boats and tugs, and that's what they're using below the water line. Because the paint is so thick, I decided to roll it on vs using reducer and trying to spray it. I did reduce the paint 5 % by volume which gave it a better consistency and seemed to help roll it on. As long as I did not allow more than 24 hours between coats, a chemical bond between coats would occur and no scuffing was needed. Three days gave me three coats. Once the boat is off of the cradles, I'll blast where the cradle was, scuff the rest of barrier coat and apply the anti foul.

Because this paint is designed for below the water line, and the rest of the primer on the boat is not for below the water line, I painted the barrier coat six inches above the water line. My plan is to bring the top coat green hull paint down on the barrier coat by two inches which will allow me to bring the anti foul bottom  paint four inches above the water line. The boot stripe should start at the six inch elevation which will hide the parting line created by the barrier coat. The boot stripe will not get painted until after launch once we are able to see just where the water line ends up.

Having the bottom painted allows one to better see how the boat will sit in the water. With this job finished and all the cut up fore and aft paint work completed, painting the hull green is now a reasonable job that should take only a few hours to  as I'm only going to spray one side of the hull at a time. Scuffing, taping, and minor repairs should take a day , maybe a day and a half. The paint should be wrapped up by early June.



  1. Thank you for sharing this interesting and informative article, painting with airless spray gun will be faster and more interesting!