Hello all. I have started my weekender - FINALLY! I haven't posted for a while so I thought I would get back on board by asking a question that has been bugging me for a while. In reading through the most excellent web site of Phil Gowen's Aloha I was intrigued by his method of applying epoxy before assembly of the parts. I don't know that I would want to do this on my entire boat but for a few of the parts I can see where it would make things easier. Since I have now started my own boat construction this has been on my mind. Last fall my son and I built a small dinghy and we covered the entire hull in epoxy before the primer and paint. It made for a wonderful finish but the sanding got pretty old. I was under the impression that between coats of epoxy, or between epoxy and paint you had to sand the epoxy to a dull finish or the next coat (or paint) wouldn't bond. Is this true or not? Phil's use of encapsulated panels in his construction made me half sick thinking about all the sanding he would have had to do every time he wanted to fillet a seam. Or did I just do a bunch of sanding that I didn't really need to do?
It won't be too long before I am once again mixing epoxy. I'll sand like I did before if I have to, but man it sure would be nice to hear that I could eliminate some of the work...and dust. Thanks
(01-20-2011, 10:28 AM)David Oelker link Wrote:I was under the impression that between coats of epoxy, or between epoxy and paint you had to sand the epoxy to a dull finish or the next coat (or paint) wouldn't bond. Is this true or not?
It is not true that you must sand between coats. If you use a quality low/no blush epoxy and you recoat within 24 hours you need do nothing between coats. The bond between the coats will be chemical not simply mechanical and will be superior to a fully cured coat that is then sanded and then recoated. The real danger in all of this is some sort of contamination. If the epoxy blushes it needs to cleaned off. This does not require sanding but rather clear water and a scrubber. It washes off easilly. Other contaminants can settle on your work and may inhibit the bond between coats.
Chris has it mostly correct in that you can apply subsequent coatings of epoxy to what we call "green" epoxy. Green epoxy is that which is still uncured. Epoxy goes though stages as is dries and the green stage is the one just after it's application, from a wet gel that's no longer spreadable, to a dry to the touch, but still soft plastic. Under ideal conditions, epoxy takes a few weeks to fully cure, but is usually within 90% of it's full cure in 24 hours.
Most of the "usual brands" should be over coated within a few hours of the previous application. Hardener types usually govern the time, with slower hardeners requiring more time naturally, but if it's 12 hours past the last application, you're probably over the chemical bond time and have a blush. This assumes you're not "pushing it" like us pros do, which is using a particularly slow hardener to allow assembly and adjustment time in lower then ideal temperatures for the hardener selected. For example, I have a custom formulation of super slow for use in very hot weather, but I use it for assembly all the time. It will not even setup if the temperature is in the lower 70's, so I can spend all day fiddling with something, then heat the assembly to activate the cure. Naturally, this is way past what the back yard builder is doing.
To check for the green stage, use a finger nail and press it lightly into the fresh goo. If you make a dent with moderate pressure, it's still "green" and you don't have to sand before applying another coat of epoxy. On the other hand, if you applied your epoxy in high temperatures (80 or up) and/or high humidity (70% or up) then the likelihood of having unreacted amines at the surface is very high (regardless of epoxy brand used) and it will need to be washed off before a new coating can be applied.
Even non-blushing formulations of epoxy will blush off in certain conditions, in spite of what the label on the jug might say. It's a function of the environment, your mixing skills, the precise ratio of resin/hardener and application techniques that produce higher or lower likelihood of blush. It's not just a chemical reaction as most folks think, but also a reaction to physical conditions and application.
In most back yard building settings, you can pretty much assure yourself that the conditions will be right for some level of blush. Even in a small shop, the conditions will require washing off amine. The only way to prevent this is environmentally controlled working spaces and the use of limited exposure techniques (infusion, bagging, peel ply and sheeting, etc.).
The short of it all is you can over coat without sanding, but you have to work clean and fast. By this I mean if you're outside under a tarp then you're probably going to have everything in your epoxy, dust, pollen, bugs, ozone, hair, you name it. Unless you're applying the next epoxy coat while the first one is just barely past tacky, you probably should wait, wash and then sand to insure the surface is clean and toothed.
If you're in a garage or other unheated, unventilated, but under roof space, then the same applies, though you'll have slightly less critters and ozone, but still enough to warrant some of the glycol groups to evaporate and leave amines unreacted.
In these building environments, the use of peel ply or other surface protectants, such as Mylar or plastic sheeting can prevent much of the contamination issues as well as blush formations. Again, if you're epoxying in the rain on a hot day, you're screwed and you're going to have blush. The peel ply or sheeting seals the surface from the environment to a large degree, which is why it works.
As a rule, if you aren't sure the epoxy is still "green" then assume it's not and wash and sand for the next coat. If your fingernail makes a dent fairly easily, then it will be okay to over coat with another round of goo, but watch your conditions, because (some epoxies are worse then others) humidity and temperature will usually force your hand, unless the surface is protected from the environmental situation and temperatures and humidity are within the "range" the epoxy formulation can tolerate.
Thanks for the replies fellas.
Paul, when you say you have to wash and sand for some "tooth" exactly how much sanding are you talking about? If there is some shine still showing here and there is that OK, or must the whole surface be sanded dull?
In a perfect world the whole surface will be dull. I never use higher then 100 grit, unless I'm clear coating. 80 grit will knock down the highs pretty quickly and you'll be dull fast. Higher grits will take longer to get into the valleys so, consider a coarse grit Scotch Brite pad to scuff the area first then the sand paper. The Scotch Brite will get down into the low spots better then sand paper.
Generally, I don't try to be neat about "toothing" up a surface, just knock the gloss off and move on. If you have stipple from a roller or brush makes, then you need to get into the low spots. You should be able to make a smooth epoxy coating, which saves you a lot of effort come sanding time.
Rollers and brushes are not the best ways to apply goo. They suck up a lot, just getting them wet and you never get the last drop out of them, meaning you're wasting a bunch. If you use a roller, tip it off with and plastic applicator or rubber squeegee. You can also put your finger on the roller as you make the last stoke, which freezes the roller and knocks down the stipple it's left. A rubber squeegee is the best thing for epoxy coating larger, relatively flat surfaces. You can apply it very uniformly and as thin or thick as you want. When dry is baby's butt smooth, with an occasional ridge, easily handled in the next coating's prep. A plastic auto body filler applicator is almost as good as a rubber squeegee. It's not as flexible so you have to work heavily curved surfaces differently, but on large relativity flat surfaces, a damn good way of laying down goo.
I've been in the midst of my first full epoxy project and am enjoying it quite a lot, but I'm tiring of sanding, even if epoxy is wonderfully more mellow than polyester. I've been blocking with 80 grit or 100, then hot-coating, and breaking that with 150. I'm using the 3M vinylester putty and finishing with a quick 180 before the Brightside.
Any tips? If you do have, I suppose I should have checked earlier, as I'm almost finished...Hopefully, only a couple more days, then it's on to rigging (Yay! More fun than sanding...)
Mike
Unless you are working on raw wood, hot coating, or at least what I think you are doing, isn't necessary. The hot on hot method only is advantageous on raw wood. It improves penetration, decreases viscosity and most importantly prevents out gassing, if done properly. I use epoxy based fairing and filling putties, to gain the most peel strength. Vinylester does stick to epoxy, not as well as epoxy, but a whole lot better the polyester (Bondo and similar).
Mike,
Paul is the expert at this so about all I can do is tell you from the point of view of a first timer that mixing stuff into the epoxy and using that as bondo or gap filling glue is far better then using any other type of bondo and not difficult once you get to doing it. I use wood flour and silica for gluing where gap filling is needed and mix it up about like peanut butter. I use micro balloon powder mixed in for bondo. I have made batches that are just 2 drops of resin, 1 drop hardener and a pinch of silica or wood flour. It was to glue our trash compactor handle. That repair has held for going on 2 years so far. The point is you can make small batches or large ones depending on your need and once you get to using it you might find, as I did, that you prefer it to bondo.
One big concern I have with bondo is the pink bondo they used to fill small spots in my hydrotek plywood popped out when I bent the ply around the stringers when adding the sides. I am not talking excessive bending either, just the gentle curve along the side, and that was enough to make me not use it. The last thing I want is a filled spot that pops enough to let in water or requires a sand down, refill and refinish if it could have been prevented.
I must admit I made a typical first timer mistake and over coated with epoxy on the bottom, sides and deck. When I first glassed and coated the bottom I would set a timer and go out every 4-5 hours to add a new coat while it was still green. I put on 3-4 coats to get it smooth. It never occured to me to add a bit of filler to smooth out the weave. After that cured I sanded until it was dull then added large areas of bondo to get things fair. The epoxy bondo sands pretty easy compared to straight epoxy so becareful. After gettiing it like I thought it should look and feel I put another 2 thin coats of straight epoxy on. If I had to do it again I would use less epoxy but I do like the way it turned out and I think most beginners over do things a bit.
I took the attitude that sanding was good exercise and one step closer to finishing. That and a zen attitude of I sand therefore I am. Whatever you do ALWAYS wear a good mask. I would discover epoxy dust in my shorts when I went back to the house after sanding. A simple paper mask like they use for cold germ protection is not enough. Just my opinion.
From a technical point of view, sanding fresh epoxy requires a chemical filtration mask, as the epoxy hasn't fully cured and isn't inert yet. From a practical point of view, unless you spend a life time of sanding freshly applied goo, then a particulate mask is just fine. It's the particulates that get into your lungs and make holes in the bronchial tracks and lung tissue. These embedded bits of dried goo are like rocks and accumulate, resulting in problems like miners face with black lung. You need to inhale a lot of epoxy dust for this to happen over a long period of time, so the hobbyist shouldn't get terribly concerned.
Again as mentioned previously, polyester based fillers (Bondo) just don't preform well. Compared to epoxy, they really suck, so pick your poison well and since you're playing with epoxy anyway, for sheathing and such, why not also use it as the filler during fairing and smoothing operations.
Mike---have you guys ever thought of doing an "update" video---or a series of clips that you put on the Stevenson Projects site for the newer techniques...
I admit I am nervous about all of this since I am about to put all my parts together and then lots of filling/bondo-ing...
The video has the bondo way...but everyone is suggesting thickenned epoxy...which I am comfortable with...but would love to see some video's on it...
Okay...hope you consider!!! : ) ;D