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Changing the color in general is a bad idea because any scratch will immediately show the old color. For this reason alone I always tell people to stick to the existing color or a color very similar to the old one. In your case that would mean sanding the blue until its gone. Not an easy job but Winter has just started. 😉

I would use a belt sander. It is fast but it has a risk of sanding too hard, especially if you hold it on one spot. It should be moved continuously, movement should stop only after the sander has been lifted from the surface. When nearly done switch to an orbital (or oscillating) sander. All difficult spots are done by hand as is the final sanding. After the blue has been removed you may want to apply some filler on spots where the belt sander got its way or to repair damages. Sand with gradually finer sand paper until grain 240 or even finer. Only when the surface is perfectly smooth (feels like a baby’s bum) the boat is ready for a new coat of paint.

I would use a two pot paint and against all that is written on the pot I would add some thinner. Two pot paints need a xylene based thinner but watch out; most thinners sold at DIY shops are toluene based and totally unsuitable. Add as much thinner (usually 5% to 10%) until the paints feels good and brush stripes disappear a few inches behind the brush. Then apply the paint using a brush to spread it with cross-wise strokes and a roller to finish it. If the paint is not perfectly glossy some seconds after rolling add some thinner or check the temperature. Two pot paint does not like to be applied below 18C.

You could use two pot Marine paint, for example Hempel’s but I fail to see what is so special about marine two pot paint. Modern cars have lots of plastic parts as well and use two pot paint too. If you think about what the paint has to endure on the road it must be at least as good or better then marine paint. The fun part is that car paint is significantly less expensive then its marine sibling. If you ask me, its the same stuff, but you pay extra for the word “marine”. A fellow Wayfarer sailor has repainted his boat using cart paint all the way and it still looks like new after five years.

For filler I always use standard polyester filler from my local DIY shop. They carry two types one with and one without glass fibers. Which one to choose depends on how hard and strong you want the repair to be. For small scratches I would recommend to use the type without glass. These fillers are grayish-white and can be colored with some pigment if you like. The DIY solution is to mix a few drops of the paint you are going to use in the filler. Only use the color part, not the kathalist part of the paint in this case.

Finally, visit the Wayfarer Institute of Technology. There is a good chance there are some essays on the subject to be found there. http://www.wayfarer-international.org/WIT/WITindex1.html