Thread: Fixing cracked epoxy

Sometimes you can just scuff the area, flood the cracks with epoxy, then sand smooth again, but those cracks appear deep enough that you'll have sand them down a fair bit, before another coat of epoxy will hide them.

The usual recommendation is to sand the area until you're below the cracks and then flood coat with more epoxy. You don't have to go all the way down to raw wood, just as far as the cracks are deep. If the cracks penetrate to the wood surface, then you might get away with sanding down until you're almost there and flood coating. This will save damage to any stain you may have used.

I guess the point I'm making is, sometimes you can cheat a little and cracks disappear under more epoxy. Other times, you have to grind a lot of cracked materiel off, then you can repair the coating. It all depends on how thick the coating is and how deep the cracks are.

Much appreciated PAR. I suppose I was hoping for a quick fix to avoid some hard work, knowing deep down a short cut was a bad move. Looks like I've got some sanding to do.

The epoxy's three coats thick and looks like it's pretty much down to the ply. The cracks are so fine though that not only are they hard to see in the now fading light, I doubt that fresh epoxy would be able to fill the gaps so I'm going to have to sand back a fair way. So long as it's fixed by this weekend, which it will be, I'll be happy.
Thanks again for the advice.

It's often helpful to open the cracks a bit with a pointed tool or corner of a scraper.

You could try flexing the ply from the back when you overcoat it with fresh epoxy so that the cracks open up and fill with epoxy. Epoxy is pretty viscous so it may not work. Worth a shot though.

I've repaired similar damage to the interior wooden parts of an old Mercedes in this manner, but the difference was that I used 2 part polyurethane which is less viscous than poxy.

LOL. Done a bit of Mercedes zebrano resto myself, on a W116 350SE. I used Cabot's polyurethane single pack in an aerosol about 6 years ago. Worked a treat. 2K clear I've seen used too with great results.

I set to task on the boat yesterday morning after your post PAR and coincidentally tried flexing the ply a bit too like you mention woodeneye just to see if there was a chance of opening the cracks to flood fill, but I made the cracks track farther. I half expected it. Not only that, I noticed long cracks I hadn't seen before in the bottom panel. Longish ones that run parallel to the veneer's grain and very hard to see. So I bit the bullet, sanded back, two thinner coats of epoxy and have begun to undercoat the hull on it's way to a gloss white finnish.

I am still a big fan of timber finish but we had in mind to paint this canoe due to the blemishes in the ply. In the end the decks, gunwales and inwales will still be clear finished but I just want to avoid the chance of water seeping through any subsequent cracking and reaching the ply. Hopefully, the paint will provide that flexible barrier if cracks do reappear through use and abuse.

Originally Posted by Canoath

... The epoxy's three coats thick and looks like it's pretty much down to the ply. The cracks are so fine though that not only are they hard to see in the now fading light, I doubt that fresh epoxy would be able to fill the gaps so I'm going to have to sand back a fair way. So long as it's fixed by this weekend, which it will be, I'll be happy.
Thanks again for the advice.

Is this an argument for not putting a relatively thick layer (three coats) of epoxy on these ply boats? If the epoxy wasn't there or was only enough to fill the grain before the paint or varnish went on, it would have presumably just flexed, and repair of any scratching would be much easier. We have John Welsford's opinion over on the Wooden Boat Forum that boats last better with paint directly on the timber, for exactly this reason.

I'm curious to hear what PAR and Mik think on this, also the benefits/downsides of epoxy under varnish, which JW doesn't address. (I think he's on record as saying the best varnish is six coats of white paint).

Ian

What brand of ply was it?

You can always stress any material to a point where it will break.

But a long time ago I saw some University tests of dropping steel balls onto ply samples coated with different brands of epoxy.

The leading brands had very little damage ... but the cheaper ones showed very considerable damage with the balls dropped from the same height.

It DOESN'T at all mean that this is an inferior resin. It might be a good one that has been hit harder!

It probably won't recur.

if it recurs in the same place it might be necessary to remove the epoxy down to bare wood in the area and then build up the coats again. This can be a spot repair.

But the simple way is best to try first.

Michael

Michael.

Originally Posted by IanHowick

Is this an argument for not putting a relatively thick layer (three coats) of epoxy on these ply boats?
Ian

I wondered the same thing. I read a while back on one of boatmik's epoxy pages an FAQ like "how much epoxy is enough", A: "not as much as you think". It was in the back of my mind but must have forgot to retrieve that info when all I could think about when applying poxy layers was equating physical protection with thickness. I ended up with not just three layers but three thick layers on the bilge and bottom panels. The bottom's panel which showed the most or longest cracking had the thickest layers of all thanks to be being flat and with the hull upturned the 3 epoxy layers were poured on and spread about. So my opinion now, the epoxy was too thick to cope with the small amount of flex encountered in mid panel areas. Glassed joins, bow, stern etc, all much thicker again but no cracking there of course due to their rigidity and reinforcement.

I can't stop there and not acknowledge other questionable methods and materials in my boat. To answer you question boatmik, I did everything on the cheap due to time constraints and availability of materials locally which saw me settle on Diggers brand resin and 6mm bracing 3ply. Gunnersons, CCA bond, I can't remember what else was stamped on the ply. I had to buy 4 sheets instead of 2 to find enough knot free, join and void free veneer. There's just as likely to be hidden voids and knots in the middle ply of parts I did use, creating a flexible line or spot in a panel. Although everything seems nice and strong at the moment, we'll have to see if any problem areas arise.

Ian, I came across that reference a while back about paint making the best varnish and decided to go down that route on the hull and I'm really happy I did. Looks and maintenance wise it suits this boat and what we're using it for IMO. Thanks for that link to JW and kenjamin thoughts, great stuff. I have to admit I think this bit sums me up,

"I have seen people try to rationalize using inferior ply with the intention to make up for it by adding epoxy and fiberglass. That's a strategy that always ends up costing far more than it would have to just use the good stuff in the first place."

The 3 coat recommendation for straight epoxy over raw wood, is to stabilize the moisture content. 3 coats will give you about 15 mils film thickness (about .015" or .38 mm), which is generally accepted as the (minimum) point at which you've stopped moisture vapor ingress, sufficiently enough to prevent movement, rot and other issues. Most formulators would like to see 20 mils (.02" or .5 mm) film thickness, but tests have shown 15 mil will get it done.

This relatively thin coating will harden the surface slightly, but not a lot. You will also gain an improvement in abrasion resistance, but again not much. There are also other gains, such as temperature tolerances, etc., but a 15 mil ( 3 coats) coating isn't going do do much more then this. It just seals and stabilizes.

If you apply 2 coats of epoxy (10 mils), then a light sheathing (set in epoxy), you'll still get the moisture stabilization (very desirable), but you'll also greatly improve several other physical properties as well; like abrasion resistance, impact (depending on fabric and resin type), stiffness (depending on fabric and resin type), etc., etc., etc.

Simply put, we coat wood with epoxy to lock down the moisture content, thus stabilizing the material. It doesn't shrink or swell, which makes it reliable dimensionally, as well as prevents little beasties (rot, mold, etc.) from getting at the wood fibers and having a buffet.

There's a good argument to be made against epoxy under varnish or other clear coatings. It can be said epoxy will just add to the maintenance of these clear coatings and in some cases I agree. On the other hand, it must also be said that non-coated pieces, will not be dimensionally stable and subject to all the issues this can bring to the table. In a nut shell, if I'm working with a traditionally built boat, I'll shy away from epoxy under clear coatings, but if it's a modern goo'd build, then epoxy makes much more sense and I tend to encapsulated as a mater of course on these types of builds.

Good discussion!

Canoauth - I can understand completely where you are coming from. The inferior ply I refer to is some real bargain basement stuff with bad faces and voids. I don't really mean reasonable quality exterior ply which can be quite nice sometimes.

PAR - completely agree with the point ... Modern boat then modern materials and methods. Trad boat then more traditional building approach. Only exception I can think of is maybe ply decks on trad boats because they add so much strength and stablity to the rest of the structure and there is littlle harm in gluing them in place. With trad materials on modern boats, there can be an argument for initial cheapness which is a reasonable one ... but the inevitable roundabouts are in terms of maintenance and longevity - particularly light portable boats. Comments?

Best wishes
MIK

Well, after PAR's last post I can definitely say that my 3 layers were too thick. The bottom's panel surface area would have easily been coated 1mm/40mil or more thick. Even following the rub back and 2x recoat it would still be around 20mil. Although I'm positive my epoxy application has improved in speed and uniformity, it would still be on the generous side.

Whilst reading some GIS threads last night I got a similar sense that my initial method of coating, pouring and pushing epoxy around, was too generous. I'm looking forward to the next project some time this year to try a more "less is best" approach. I've no doubt it will lead to a better finish for both painted and clear finishes. My Eureka photos don't show how rough my surface is, but up close it's exterior is uniformly undulating. nothing wrong with it, it just adds patina and I'd go nuts trying to maintain a new car like duco on a canoe.

The epoxy is far too thick on my GIS as well. I was pretty sure I was putting on too much when I ran out and had to buy more. Weighing my finished hull confirmed it -- at least 15 pounds heavier than design weight. If I ever build another boat, I will know to spread it out much thinner.

The best method of spreading epoxy I've found is a squeegee or plastic applicator, like the type used for auto body filler. You can apply it as thick or thin as you like and very uniformly. Most who use chip brushes and rollers can't get the level of control had by a squeegee. A brush just makes a pile, that's slightly dragged in each direction. A squeegee smears the puddle of goo in a uniform film.

On the first coat on raw wood, I use a good bit of pressure and smear the goo fairly thin. When it kicks off, it's decidedly dry looking, which means the pores are partly filled. The next coat is slightly thicker, looking to top off the pores and partly fill the grain, particularly on coarse grain woods, like mahogany and Meranti. The last coat is applied without a lot of pressure, but with an eye toward a uniform thickness.

If you're going to sheath the surface, just two coats are necessary. The third coat will come when you apply the sheathing, while wetting out the fabric. The only time I deviate from this routine is when I'm attempting to level a surface with goo, under a clear finish. This takes some serious skill, but you can fair a clear finished surface, with varying amounts of epoxy thickness in the low spots. Naturally, if you need to fill deep hollows and depressions, you'll have some discoloration, with the thicker epoxy areas, but if you were good about fairing and smoothing before the goo goes down, then it's not noticeable by anyone except a pro with good eyes.

With plywood decks on traditional builds, I tend to let the fasteners hold the monocoque deck surface (bonded seams) to the calins, clamps and shelves, with a bedding offering some give. The plywood is encapsulated, but installed as a separate unit over bedding, not bonded. I do this because I might have to remove it some day and would rather not break out the reciprocating saw to do so. The deck is still sheathed and the end grain at the edge of the plywood, if not buried in a rabbit, rolled over and covered as well. This is hidden by a rub rail, covering board or what ever the design calls for. This way you only have to destroy a small line of filler at the deck edge, not half of the sheer plank, shelf and clamp assembly in future repairs. On the other hand, a modern build would be an embalming job, with everything sheathed and goo'd down, maybe not even a single fastener.

Lastly, you have to remember what you're applying. It's a hard plastic coating, that by itself, tends to be brittle. Once you have sufficient film thickness to insure waterproofing, you're done or you should strongly consider a reinforcement (filler, fabrics, etc.) Epoxy becomes a wonder goo, once reinforcements are added. Directional fabrics are best (unidi, biax, triax, etc.) once you get over the cost of the high tech stuff like Kevlar and carbon. Cloths are next up on the list, with multiple layers of lighter weights, being stronger and better then one single, but heavier weight fabric. The rovings are next, but these are really bulky and not often required in sheathings. Mat doesn't do much for epoxy layups and shouldn't be used generally, though a thin veil can help stop print through under paint. Finally are the non-'glass materials such as polyester and acrylics. These fabrics only do one thing, offer a huge abrasion resistance barrier. They also drink resin like I do beer on a Saturday night, but they are really tough when cured (unlike me).