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 thats what theyre 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, Ill 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 Im 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.

Cheers

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Didi 950 Build Bottom Skin Panels

Mike Vermeerschs amateur boatbuilding project of a Didi 950 is moving on to the next stage, with bottom panels fitted and work starting on the side panels. Mike is building from a plywood kit that we supplied to him and reports that the fit is good. The kit has jigsaw joints on all large panels, making them easy to assemble either on the floor or on the boat.

Didi 950 with all stringers installed.

Two of the stringers each side, at the junctions between flat and radiused skin areas, have plywood doublers attached to serve as backing pads to the joint. I call these the tangent stringers. The doubler on the upper tangent stringer is in process of being fitted and can be seen running forward from the transom through to the third bulkhead from aft.

In the photo below, the transom has been fitted, followed by the bottom panels. These panels each have two transverse jigsaw joints and join each other at a centreline butt joint over the plywood backbone. In the photo the jigsaw joints have temporary battens over them to secure them while the glue is setting.

Bottom panels fitted.

Mike has also started to dry-fit the side panels ahead of gluing in place. At the right of the photo the forward lower panel can be seen and which will continue through to the transom. The lower edge, as seen upside-down like this, forms one half of the chine. The other half of the chine will be formed by the upper side panel.

Today Mikes 300lb brother decided to test the hull stiffness of this partially-built Didi 950 . He climbed onto the bottom of the boat and jumped on the bottom panels. They passed his improvised test but I think that I would have recommended that he wait until the whole hull was skinned before doing such a test.

Mikes 300lb brother tests the Didi 950 hull stiffness.

The broad stringer that shows on this photo is the upper tangent that will join the lower side panel to the radius.

To see our other designs, visit http://dixdesign.com/ .

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