epoxy or alkyd enamel for barrier coat?

[andy]

We are redoing the bottom of our ketch and read recently that don casey recommends a couple coats of alkyd enamel as better than epoxy barrier coats.

does anyone have experience with this?

Andy N1ZXD
CD30K #2
Watermark II

[Dan & Pat]

Andy,

I'm not a chemist, but what I've learned by making mistakes when it comes to bottom paint and barrier coating would fill a ship's logbook if I ever get around to writing one .

First and formost, the barrier coat and the subsequent bottom paint that's applied to it have to get along well during application as well as once they hit the water. Most paint manufacturers have done extensive research on the proper preparation and application of their product, and the label will spell out what should and should not be done. Obviously, they want you to use their brand name products, that's called marketing.

What many people fail to comprehend is that products with different bases, although they might have an easier application process have to create chemical bonding between each other. For example, gelcoat doesn't stick to bottom paint very well because of the release wax that's put into it when it goes onto the mold. Therefore, it has to be barrier coated with something that will bind to it.

The barrier coat, if its alkyd (oil based) usually loves the gelcoat and they become friends immediately, to the point where it is almost impossible to get it off (oopsey!) when you get some where you didn't want it or get it too thick. It goes on pretty easily though.

Epoxy based barrier coating on the other hand is nasty stuff to apply. You have to be careful to mix the components in the correct proportions, make sure the temperature and humidity levels are within acceptable ranges, and most importantly, apply it within the acceptable pot life (time before it begins to cure). On the bright side though, it is a more "water proof" seal, so it is less likely to promote blistering even after many years. Not all bottom paints like epoxy paint though, so you need to have your ducks all lined up before jumping into the job. Know what paint you are going to use, and read that label too. Some bottom paints recommend that you apply them over a barrier coat which is not fully cured, to promote a better bond. I didn't read that far ahead on the label once, and I ended up having to re-sand the whole hull on a 26 footer just to rough up the surface before applying the bottom paint.

So, the bottom (painting) line is, read the label, read the label, check this post for other opinions, read the label . . .