Epoxy Sanding

I am building an Eastport Pram and have finished applying epoxy to the hull. The instructions say I should sand the epoxy before applying a primer. I am wondering how much sanding I should do and what am I trying to attain with the sanding?

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RE: Epoxy Sanding


that's a great question.

first, to confirm, it sounds like you are going to paint the boat.  becuase if you are not going to paint the boat, no primer is necessary.

the purpose of primer is to get a very smooth surface to give you a corresponding smooth, beautiful paint finish.   however, primer is not necessary for painting....and the paint (at least Britesides) will bond very well to sanded epoxy.  in fact, the bond between paint and epoxy is stronger than paint to primer to epoxy....so the stronger the finish you want, the less primer that shoud be there after you sand the primer.  a boat that has primer as part of its construction, if properly done, looks like almost all the primer is sanded off.

so with that in mind, i would say that what you are trying to do with the sanding is get a basically clean, smooth surface.  primer is used to fill the tiniest of gaps, pinholes and scratches... if you need it at all.  primer is not like bondo ...capable of covering signifcant imperfections.  and that is why i said, above, when done properly, it looks like it is almost all sanded off.

that said, over the years, i have basically taken primer out of my methods and just sand my epoxy finish to a level of fit and finish that gives me a great paint job that will last.  my last primer boat (about 15 years ago), i ended up refinishing and sanding it all back to smooth epoxy becuase the places that had a lot of primer easily chipped and scratched off...requiring the boat to be refinished to bring it back to looking great.

hope that helps.....i am sure others will share their opinion as well.



RE: Epoxy Sanding

As Howard predicted, here's another opinion. First off, I agree with everything Howard said. Second, since you're painting, I would suggest that you use an epoxy/phenolic microballon mix to fair and smooth the surface. The stuff is light, easy to sand and when sanded with a longboard produces a ready-to-paint surface that epoxy-compatible paints love without chipping the way primer does.

Just slop it on, wait until it cures and have at it with the long board. Just as with primer, most of it will sand off. Only the low spots will be filled and the surface will be as smooth as the sandpaper grit allows. Clean off the dust, wipe it down with denatured alcohol a few times and you'll be ready to apply paint.

The stuff really is easy to sand. I sanded an entire 18-ft kayak by hand in several hours, starting with 80 grit and working through to 220 as the layer got thinner. It's also somewhat astonishing to see just how wavy glass and epoxy application really is. On bright finished boats you never get that view since clear epoxy is all the same color.

Have fun,


RE: Epoxy Sanding

just wanted to add to Laszlo's comment.....and that we are in mutual agreement.

in  my note, i referred to 'bondo' and i would say that laszlo's note is about how to make a marine grade 'bondo' that is very easy to sand and can also act as a primer if you need it.  

the main thing is that bondo (e.g., the microballoom epoxy) mixture can fix things like waves or other significant imperfections like depressions or major gap or unfilled weave.  primer is just a surface preperation and cannot deal with these kinds of situations.

so if you have fare surface with no signifcant imperfections, you can just sand your epoxy smooth and paint it, if you want to address imperfections as described above, use the bondo approach and then paint.


RE: Epoxy Sanding

   Thank you for the suggestions. Unfortunately I was not clear in my question. I am wondering how much to sand the epoxy before I apply the primer. Should I just try and rough up the epoxy or trying to get all the runs and drips etc. smooth?

RE: Epoxy Sanding

   Sand off the runs and drips smooth prior to applying primer unless you want the runs and drips as part of your final product.   the final surface prior to primer is typically sanded to120 grit.


RE: Epoxy Sanding

   If it helps, I have used Abranet open mesh sanding discs made by Mirka to sand epoxy.  They seem to work much better than typical sanding disks. 

RE: Epoxy Sanding

   Just don't sand through your epoxy into either wood or fiberglass in the case of surfaces which have glass on them.  If you are getting close or break through, stop, recoat that area and let it cure.  That's where the microballoon mix comes in handy for painted finishes.  You can easily build up to fill low spots then quickly sand down to a mid point where the high spots are gone but you haven't gone through the epoxy sealer.  I did this on my SUP I did recently because my puzzle joints and 'glass work was not the best.  So, I mixed up the epoxy/microballoons and slathered it on then sanded down.  Yeah, there's a little extra weight in that solution, but it doesn't bother me.  And the final finish doesn't show all those ripples and bumps now.  

Oh, and I'd wait extra time before sanding the epoxy.  The "book" cure times are minimums, in my opinion.  Try sanding a spot or two and if the paper gums up fast, stop and wait more time, preferably with the boat in a warm to hot area.  And use a respirator.  Epoxy dust in lungs is no good.

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