Jump to content

Epoxy Tricks


Howard

Recommended Posts

My apologies in advance for posting a landlubber question on this bluewater forum.  But here goes:  Does there exist a clear UV protection that will serve as a topcoat over epoxy?  Even if it has to be recoated every year.  I'm talking for patio tables here folks, not boats.  A simple yes or no is fine, but if it's yes, a brand and name (of the product of course) please.

 

The answer probably is somewhere in the foregoing pages here but I can't find it. 

Link to comment
Share on other sites


There are many choices; traditional varnish, modified varnish, single and multiple part polyurethanes are the usual choices (there are others). Any clear coat over epoxy is a difficult coating to live with, because these coatings have to sacrifice so much, to offer the clear portion of the product. I assume this is over wooden table tops, etc. Regular varnish will offer the least protection, but does have a traditional look and application. Modified varnishes are a little better, but not by much. Single part polyurethanes keep cost down while offering more protection and hardness, but you have to stay after this stuff or repair becomes painful. The same is true of multi part polyurethanes and acrylics. These are the most durable, most costly and the most difficult to repair. So, the answer is" it depends" on how much care the piece will receive. If you're likely to coat and forget it, then traditional varnish is the way to go, as you can repair it fairly easily. If you want really durable, high gloss retentions, hardness, etc., then the polyurethanes are the way to go, knowing you'll need to stay after this type of coating as if you let it "get away" from you, you'll have to strip it all off and start again.

Link to comment
Share on other sites

Thank Paul!  Great summary of what I think was said over the previous eight pages :)


 

 

OK, I'm thinking single part polyurethane. 

 

 

"Single part polyurethane keep costs down while offering more protection and hardness, but you have to stay after this stuff or repair becomes painful."

 

1) Does this mean only regular cleaning and maybe waxing or something more?  Such as a light sanding and recoating annually?

 

2) Regarding the "pain of repairing", I assume this is because the uv damaged epoxy is so hard to remove and then replace?  These are small (30", six sided) tables, relatively easy to sand with power tools, i.e. no curved surfaces, and the tops only.  I will probably make powder coated metal bases.

Link to comment
Share on other sites

The regular oil varnishes can be sanded, feathering into surrounding areas and more varnish "flowed" into the repair area for a seamless fix (with some practice). This assumes the varnish was in good shape, just damaged, by abrasion, a dropped winch handle, etc. If the varnish is allowed to go "too long" it will crack, wrinkle and split with wood movement. In this case you have to sand it smooth again and add several more coats, but it's still repairable, unless really brittle and left way too long, between maintenance coatings.

 

This isn't the case with the hard acrylics and polyurethane varnishes and clear finishes. If you catch scratches and minor ding early, you can sand and feather the area, but you'll have a harder time "flowing" more coating into the area and still have a seamless repair. It can be done, but it's harder to do well. The real draw back to these harder finishes is they will need to be completely stripped off the surface and you're starting over with new coatings, if you let them get away from you, in regard to care and upkeep.

 

I like the polyurethanes, but I'll keep up with them, so I don't have to strip them down. Any clear finish over epoxy requires careful attention or UV will damage and darken the goo. Usually it will also cause some damage to the underlying wood too, so you'll cuss and drag out the various bulk removal tools. In a nutshell, the regular varnishes don't last as long and can be harder to apply with drying times being long too, but they're easier to repair and refinish. The polyurethanes are more durable, easier to apply, but less tolerate of neglect and repair is more difficult.

  • Like 1
Link to comment
Share on other sites

  • 3 weeks later...

I've read several things about working with epoxy in cooler temperatures and the need to keep things warm.  What about in really hot temperatures?  I expect to be working with epoxy this summer where the highs will be over 100 and the overnight low will be upper 70s.  Any special considerations for epoxy in these higher temps?  Should I buy the slow or fast hardener, or both?

Link to comment
Share on other sites

The only real issue when hot is working time as John mentioned.  Definitely slow hardener.  And the sooner you get the mixed epoxy out of the cup or bucket you are mixing in the better.  Better to get it spread out, even if you have to work it out more after you pour it.  Heat is a catalyst to epoxy cure, and epoxy cure creates heat, especially when in a 3 dimensional shape, like in the pot.

Link to comment
Share on other sites

Dave, epoxy is "activated" not a catalyzed reaction. Though heat is a byproduct of the reaction, it's because the two separate molecules (resin and hardener) are reassembling into a new, different molecule (a fairly violent process). Unlike polyester resin that is a catalyzed molecular reaction, whereas the two molecules just intertwine, but do not become a different molecule. In fact, the hardener just remains in suspension in the cured matrix. The resin molecule is changed (altered), but the hardener is a separate item that just causes the reaction to occur.

 

Just to get technical . . .

 

Dave is right, you need to spread out the goo as soon as practical in warm weather. I mix the resin/hardener in a big (12x18) mixing tray, so it can self level as soon as it's put in. Most slow epoxy formulations aren't slow enough in the tropics (90+ weather), so you need super slow. West System 209 is very slow, about the slowest of all the slows commonly available. Marinepoxy from Bateau.com (or DuckWorks) is fairly slow, but not as slow as 209. In it's favor, it's a non-blush formulation, while 209 will blush. Once you get into the high 90's and low 100's, you have to take steps to lower the application temperature. One easy way is to use those misting fans, that squirt water into the fan air stream. Of course, this will dramatically raise the humidity, but if your weather is like mine, it'll probably already be 115% humidity anyway. Humidity will cause blush, so you'll be washing after the cure, but you'd do this anyway, right?

 

Take a look at my "Building Tips and Tricks" link, where I cover epoxy use in a little more detail.

Link to comment
Share on other sites

One thing I have done is put the mixed epoxy into a metal paint tray, then set that tray into a second one with crushed ice or ice cubes in it.

 

I glassed the hull on a Princess 22 (Pilgrim) by myself in a shop that was at or above 100 degrees. Used B and B epoxy with just slow resin.

 

Still can't mess around, and still need relatively small batches.

Link to comment
Share on other sites

I avoid metal trays, simply because of the thermal convection properties of metals. Besides being strong, there's a reason metal is popular for frying pans, they conduct and hold heat well. Of course, a bath in a cool tray helps a lot, though I don't like to have viscosity change much from try to work. I do use a small fridge to hold a mixed batch while I move or setup something I should have done previously. With experience, you just learn how much you can "play" with depending on the current conditions.

Link to comment
Share on other sites

He mentioned the overnight lows in the 70s. Since no one has picked up on that I will take a stab at it.  You can always work in late evenings or early mornings.  Actually, if you are 1st coating bare wood with epoxy the ideal situation is decreasing temps.  Unlike FL where we have H 95 L 80+ in the summer you may have ideal temp variance. Coat epoxy in evenings and temp decrease will suck it in.  If I remember right, Rob White wrote about doing major coating operations starting evenings and going well into the night with his air conditioner turned on also.

Link to comment
Share on other sites

  • 5 weeks later...

To those reading this later on, Chick got his answer. With ply in the 1/4" range, or thinner, best to use tape. With thicker plywood, scarfs made with finger joints, tape is probably not needed. If a person was going to later glass over the entire panel, including the joint, that serves a similar purpose as the tape.

Link to comment
Share on other sites

I thought the topic of scarfing plywood had been covered before, but apparently not in this thread.

 

There are several ways to do this, but most have evolved into one of two methods......scarfs (scarphs) or butt joints. Since both are accomplished with epoxy, this may be an appropriate place to mention it.

 

Scarfs seem pretty straightforward. Decide on one of many methods to cut an 8:1 bevel on one end of each sheet of plywood you want to join, wet them out, apply thickened epoxy to the surfaces, then align them and press them together until the epoxy sets. The large glued surface area should be a strong as the plywood itself and some say it will have better bend characteristics, similar to the plywood itself, with a fair curve and no flat spots. About the only evidence of the scarf is a straight line in the joint. Although this seems pretty simple, there are several things that can go awry. Basically, anything that deviates from a perfectly flat square bevel that ends in a feather edge has to be compensated for, and since the panels do not align square to themselves, so you have to find a way to do that as well. Lastly, if length is tight, you do lose the width of the scarf from the overall length of the two panels. For instance, joining two 8' sheets of 1/2" plywood leaves you a single panel that is 15' 8", not 16'.

 

For this reason, the most often used alternative is butt joints. These come in two forms. The older style was plywood butt joints, which placed a plywood butt block on one side over the joint. This was strong enough but left the bulky plywood butt block in the way. The most common alternative to this is the Payson butt block, which uses fiberglass tape on both sides of the joint. This is apparently as strong as any, is simpler than cutting scarfs, the downside being the "bump" left by the layer of glass tape on the surface. Some say butt blocks of both types do not throw as fair a curve on bent panels. I don't have enough experience with that to say, so won't go farther than to mention it. I can't think of too many areas where that would matter except for topsides where you might get a long visual look at a panel and more noticeable still if painted a dark color or with high gloss paint.

 

One of the original methods used to fair out the glass tape was to cut in a channel at the joint for the tape to "nest" in. For example, you used a plane or power planer to cut in a groove the same depth as the thickness of the tape used, which depended on the weight of the tape. That proved to be a lot of work and a complication that wasn't needed, so you don't see much about that anymore. One alternative that might work, if you are going to be glassing the entire panel, is to only apply tape to the side that will not be glassed over. That will protect the joint enough to survive construction and the glass over the joint later serving the same purpose as the tape. Tape on both sides, then glassing would be stronger still. Remarkably, some say that two edges simply butted together and not taped at all may not be strong enough to survive a turnover, let alone any future use. That says a lot about the strength of the glass tape that manages to hold these together, surviving the forces of tension, shear, bending, etc.

 

It is possible to do multiple butt joints at the same time by stacking them. Put down plastic, and wetted out tape, then center the plywood joints over that, apply more epoxy and tape, cover with plastic, apply wet tape, butt plywood, top layer of tape, plastic, etc. The downside to this method is the need to apply clamping pressure to the entire stack to force the edges flush (weight of plywood layers helps with this), but also keeping the butted edges close to each other and aligned. Excess epoxy is forced out and tends to conform to any puckers or ripples in the plastic used to keep the stacks from sticking together, so these have to be faired out later on. Also, with excess epoxy, the tape tends to "float" on too much epoxy, leaving a larger bulge to fair out as well.

 

My experience has shown that when doing the glass tape butt joints,  the best method for me is to slow down and tape one side at a time, with for me does a better job of laying down the tape (right amount of epoxy) and if using tapes with selvage edges, I'll snip the selved edge of the tape as the epoxy is starting to kick, leaving a clean edge and very little fairing to deal with.  When doing it this way, the concern is to have a flat panel below (do this butt joint on top of another sheet of plywood below, with plastic between them to keep them from sticking), but also there needs to be something pressing down on the edges from above, such that the entire edge of the butt joint is flush and flat (vs. a segment that might be puckered up). That something might be weights pushing it down, or even finish nails or drywall screws (all back as far from the joint as needed to keep them clear of the epoxy).  If you can't get the edges to lay flat by any of these methods, better to put a flat panel on top and weight it or clamp it to force the entire butt joint flat and flush. Better to have to fair out something later on than having a segment puckered up.

 

I'm sure others will have ideas. My personal opinion is that if there was a clean and simple way to cut in perfect scarfs, that would be the preferred way of joining plywood. Other than a circular saw jig that can only be used for thinner plywood, I'm not aware of anything "simple". A lot of guys build jigs for routers, etc, and while simple once they are built, these are not as simple as butt blocks.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.