Hi Creekboater, 

took a look at your picture and see what you mean.  also get being tired of sanding.

primer is really for filling pin-hole voids for an ultra smooth paint finish....not for filling cracks and other larger gaps.  primer also works with paint (does not work with a varnished finish) and it tends to make the paint more fragile then not using the primer.  primer is also particularly problematic below the water line....where its not supposed to be used. 

so if you only had a choice between primer and another coat of epoxy, go with the epoxy.  you can apply epoxy with an automotive style spreader and just work along the edge where you have your gaps and focus your sanding there.  you don't need to redo the entire bottom.   

another option is to add some micro-balloons to epoxy to create a marine grade 'bondo' and use a spreader to smooth it into those remaining bumpy areas.  this is pretty easy to sand relative to straight epoxy and will take care of this easily.   you would need to have or acquire some microballoons.  you create a paste about the consistency between mayonaise and peanut butter.....and then just use the spreader to spread it into those areas.

i keep micro-balloons in my inventory of stuff.....so that's what i would do based on what i see in your picture.  but paint primer would stay off my list as a way to address this.

h

Looks to me as if you need fairing compiund. Those bumps are way too big for primer, which is meant to fill pinhole-sized imperfections. You could fair them with unthickened epoxy, but that's expensive, heavy and harder than sanding. It's what you have to do for a varnished finish, but under paint you can use fairing compounds.

You can either mix up your own from epoxy and phenolic microballoons or use a pre-mixed product. CLC sells the microballoons here and the pre-mixed compounds here. FWIW, I prefer to mix my own because of less waste and the ability to vary the texture as needed.

For best results, use a longboard instead of a sanding block. I made my own from a piece of 2 3/4" x 24" x 1/4" plywood scrap and a pair of 6" x 1 1/4" dowels for handles. An old broomstick can also be cut to make the handles. Glue one handle standing perpendicular to the board at each end. Now you can apply a 24" long piece of adhesive-backed sandpaper on the side opposite the handles.

To get rid of your bumps, smear the bumpy area with fairing compund, let it thoroughly cure and use the longboard to smooth the area. Just as with primer, you'll probably sand most of it off. Repeat as needed.

The longboard "averages" the bumps over a long distance so that you are always sanding the highest spots and not getting caught in the low spots. This means tha all your sanding is effective so you spend less time getting a better result than you would with a sanding block. The fairing compund, while plenty hard enough to make a good substrate for your paint is much easier to sand than unthickened epoxy, so you spend even less time sanding. I was able to take my 18-foot kayak hull from unfilled weave to ready to paint in well under a 1/2 day of hand sanding with a longboard and fairing coumpound.

Finally, you might want to consider fairing your entire hull if you're going to paint it. Even with all the sanding you've done, you'll be surprised how much smoother it will get with a longboard and fairing compound, Smoothness is the basis for a high-quality paint job and with a longboard and fairing compound it's actually pretty easy.

Laszlo

Ninja'd by Howard.

Ninja Howard and Laszlo,

Ok, short story I going to order some microbaloons from CLC and build me a longboard. I'm dying to (not necessarily be finished) but to see this boat complete. I can't wait. But ya'll have convinced me of the right thing to do. I'll regret it later if I don't.

Long story, I can't tell you how much I appreciate the time you took explaining WHY. I have read manuals, books, CLC tips, etc. but there's just something about having your particular question addresssed so thoroughly. Not just helps me in my current decision but I plan on doing more boats (Pocket Ship next) and ya'll were extremely helpful in my understanding of this aspect of finishing is done in general.

A huge thank you. 

Laszlo,

You mention going from unfilled weave to paint ready in half a day using fairing compound. From your previous comments and posts i assume that means 2 fairing coats you're sanding down in half a day. 

So my question...how are you determining when properly cured? I hear of gummed up paper if you don't wait long enough. Maybe you just sand a little and, if gummy, just...wait longer?

Creekboater,

That was half a day of sanding time only, not total time. It was on my WR18, my latest build, and it was the painted portion of the hull. The varnished deck took longer since I couldn't use microballoons under the varnish. For that I did the conventional sequence of multiple coats of unthickened epoxy and lots of sanding with a ROS.

I left the weave unfilled while I completed the rest of the boat, then applied a single very thick coat of epoxy/microballoon mix to the area that was to be painted and waited for it to cure. It was an ugly coat, bumpy and textured with a couple of runs, but it was thick enough to fill the weave and all the low spots.

I don't remember exactly how long it took to cure enough to sand. It was at least one day, possibly 2 or 3. I gave it the thumbnail test (see if you can dent it with your thumbnail) and also did exactly what you said with sandpaper. I couldn't dent it and it produced dust, not gumballs and the paper was clean, so I was good to go. If either test had failed, I would have tried again the next day, just as you said.

Since it was such a thick coat, I started with #80 paper on the longboard (be real careful if you use paper this coarse) and kept at it until I had a smooth surface all over. The wood and glass were still completely covered. Then I switched to #120 and took it down to where I could see islands of glassed wood in a sea of fairing compound. At that stage I switched to #220 and took it to the end. I stopped sanding when I could start to see the weave pattern. I tested to make sure that I hadn't gone too deep by wiping it with denatured alcohol to see if the weave disappeared when it was wet. That was the stopping point. Weave that doesn't disappear indicates oversanding. To fix it, brush on some epoxy, wait for it to cure and continue.

When the sanding was done, the hull looked like large areas of glassed wood with a purplish tinge with some areas of darker purple running between them. The colors varied depending on how thick the remaing fairing compound was. While the color varied, the surface was very smooth. The purple tinge is caused the the very thin layer of compound that's left when the weave is filled. Some places it looked like waffles with jam in the squares, others it was just a haze. There were even some spots with no visible color. Those were where the epoxy had been thick enough to fill the weave. It was all very blotchy, just like a properly sanded primer coat.

Here's the final result.

Good luck,

Laszlo

   Thanks Laszlo. Boy, i was really wondering how you got two fairing coats down and sanded in half a day. I was eager for the trick.

I made a couple of your longboards and they are nice to work with. The 2' was too long for my jimmy skiff but I made a couple of shorter ones. 

I think i'll be ready to paint today finally. As an experiment i'm going to note positions of a few spots that I've just given up on. Curious to see degree of how things come through the paint. 

Thanks again for all your help.