Question about sanding between epoxy coats

Hey All,

First time poster/boatbuilder here. I am building the Passagemaker from a kit. I am still very early in the build, but it is going pretty well so far.

Puzzle joints are glued together and I am in the process of coating strakes 2, 3, and 4 with two coats epoxy, as per the manual. Just finished my first coat on one side of the strakes. Used a foam roller and foam brush to tip. Didn't get too many bubbles, but the surface after 24 hours is "pebbly", kind of like sandpaper actually. I assume this is from the wood grain being raised or any other minor defects in the surface of the wood. I plan on coating the other side before sanding, but wanted to see what kind of job sanding was going to be like, so I did some test sanding on one end of one of the strakes. After a few quick passes with an orbital sander at 120 grit, the rough texture came right off and surface is smooth to the touch. However, when viewing from an angle the surface is probably 60% shiny and 40% dull, kind of in a speckled pattern. Again, it is smooth to the touch.

So my question is as follows. I understand that you want the surface to have a uniform dull finish before varnishing or painting, but what about between coats? Should I just focus on having a smooth surface to lay down the next coat on? Or do I need to get rid of all shiny spots before the next coat? I am afraid if I try to sand to get rid of all shiny spots I will sand through to wood.

My gut says that being smooth is all that is needed, but I'm worried since its been over 24 hours and can no longer form a chemical bond with the previous layer, that the next layer won't adhere properly to the shiny spots, since they weren't roughed up with sandpaper and won't have a mechanical bond? 

Also, will 120 grit give the first layer enough "tooth" to form a proper mechanical bond with the second? I read from another user that he had orange peel problems unless he used 80 grit. I am using the MAS epoxy from CLC.

Thanks for any advice you can give to a newbie!

-Steve


6 replies:

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RE: Question about sanding between epoxy coats

Steve,

I'm not an expert, but I've yet to have any part of my Peeler Skiff break, delaminate, or peel since its launching in August 2014. I can share my experience with you, for whatever it's worth.

First, we're discussing epoxy resins, so it's worth understanding how epoxies stick to things, including other layers of epoxy. As adhesives, epoxies bond in three ways: a) Mechanically, because the bonding surfaces are roughened; b) By proximity, because the cured resins are physically so close to the bonding surfaces that they are hard to separate; c) Ionically, because the epoxy resins form ionic bonds at an atomic level with the bonding surfaces. This last is substantially the strongest of the three. Second, MAS epoxy resin formulationa are non-blushing, that is the manifest no amine blush to impair adhesion between layers.

In my experience this means that the sanding one does between layers of cured epocy is primarily to achieve a smooth surface. So I've concentrated on taking down the high spots and surface roughness like runs. The next layer of resin will help to fill shallow low spots. I've used a marine "bondo" to fill deeper imperfections before the next layer of epoxy. I've also founf that an ungloved hand is a wonderful tool for guaging smoothness, though eyes are better for judging fairness.

I hope this helps.

Cheers,

Dick

RE: Question about sanding between epoxy coats

 

If you're able to sand the epoxy, you're past the point of a chemical bond. If the epoxy has "set" so that it isn't runny, but still a little bit tacky when you press a fingernail or something into it, then you can put another coat of epoxy over it without sanding. That's the easiest and quickest method since you can avoid all that extra sanding. If you were using a blushing epoxy, recoating before the epoxy cures will save you from scrubbing with soap and water between coats, which agains saves you a lot of time. I much prefer this method.

In your case, since you were able to sand, the epoxy had already cured and you will only be able to get a mechanical bond -- which is fine! Mechanical bonds are completely strong and there is no way you would ever be able to tell the difference between the two. You will need to rough up the surface - 120 grit should be fine, and actually better than 80 I would think, since you won't leave deep scratches in the epoxy (in case you want to varnish).

Epoxy wouldn't cause the wood grain to raise, so that's not the cause of the pebbly texture. I think it's a combination of imperfection in putting on the material (rolled or painted onto a surface), so will naturally be of non-uniform thickness, and the natural imperfections in the wood. You don't need to sand out ALL the shiny areas. I found that I would have pinpricks of shiny material left. If they are bigger than that, you should sand some more. Otherwise you're probably fine.

 

   

RE: Question about sanding between epoxy coats

 

If you're able to sand the epoxy, you're past the point of a chemical bond. If the epoxy has "set" so that it isn't runny, but still a little bit tacky when you press a fingernail or something into it, then you can put another coat of epoxy over it without sanding. That's the easiest and quickest method since you can avoid all that extra sanding. If you were using a blushing epoxy, recoating before the epoxy cures will save you from scrubbing with soap and water between coats, which agains saves you a lot of time. I much prefer this method.

In your case, since you were able to sand, the epoxy had already cured and you will only be able to get a mechanical bond -- which is fine! Mechanical bonds are completely strong and there is no way you would ever be able to tell the difference between the two. You will need to rough up the surface - 120 grit should be fine, and actually better than 80 I would think, since you won't leave deep scratches in the epoxy (in case you want to varnish).

Epoxy wouldn't cause the wood grain to raise, so that's not the cause of the pebbly texture. I think it's a combination of imperfection in putting on the material (rolled or painted onto a surface), so will naturally be of non-uniform thickness, and the natural imperfections in the wood. You don't need to sand out ALL the shiny areas. I found that I would have pinpricks of shiny material left. If they are bigger than that, you should sand some more. Otherwise you're probably fine.

 

   

RE: Question about sanding between epoxy coats

Craig,

I know that what you say about epoxy and chemical bonts is the common wisdom, but it is not born out by technical literature or my personal experience. Before non-blushing resin formulations were available, it was important to apply the next epoxy layer before the waxy residue created by amine-based hardeners migrated to the surface. Otherwise, you had to clean or sand the "amine blush" off to ensure that the next layer bonded properly. With today's non-blushing formulations that is no longer an issue.

As I stated in my post, above, the ionic bonds that epoxy creates with the surface to which it is bonded are its strongest bonds, with metal, wood celulose, and expecially epoxy. If one has ever had to clean epoxy off something you didn't mean to bond, one learns how strong those bonds can be. Of course, epoxy will not bond at all to some materials --- plastic wrap and polyvinyl sheeting are good examples.

When I was building 30+ years ago, we had to be very careful to maximize the mechanical bonds when laminating with polyester resins, because they had almost nonexistant adhesive properties. Epoxies are superior adhesives.

Regards,

Dick

RE: Question about sanding between epoxy coats

Dick,

I completely agree, it's better to get that chemical bond by recoating before the epoxy cures. I just meant that if you DIDN'T wait, you are still fine by sanding and coating again (don't need to sand down to bare wood and start again). In my case, I use epoxy that sometimes blushes (high humidity? Something else? I'm not sure why it does sometimes and not others). I save a HUGE amount of time by just recoating quickly rather than waiting and washing/sanding to prep for the second coat.   

RE: Question about sanding between epoxy coats

   Thanks for the advice guys, thats kind of what I was thinking, but it puts my mind at ease to hear someone else concur.

-Steve

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