Pet G or mylar film

Discussion in 'Materials' started by Tungsten, Feb 10, 2013.

  1. Tungsten
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    Tungsten Senior Member

    I've been using .020 thick mylar sheets over epoxy in some areas to get a nice flat surface on my Meranti sides,but i cant seem to get a bubble free surface.I've tried more epoxy under but still have to roll out bubbles and it seems the more i work it or roll it the more trouble it creates.I'm always left with small pits.
    What am i doing wrong?I havent tried vac bagging yet maybe thats the solution?Im only doing small areas of my sides say 2'x 1'.

    Any thoughts,thanks.
     
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    I would say, don't do it! I have never heard of this before, using Mylar to even out the resin, and especially such thick Mylar at that--0.020 is extremely thick.

    Epoxy resin is self leveling, certainly on a horizontal surface, and it will do to some extent on a vertical surface. The trick is to use a number of thin layers, two or three, rather than one thick layer. Also, sanding is an essential part of finishing, and the more you sand with finer and finer grits of sandpaper, the better the finish will look. Finally, varnish over epoxy will result in the finest smooth surface, again, many coats of thin varnish rather than a few coats of thick varnish.

    Eric
     
  3. Tungsten
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    Tungsten Senior Member

    I've had some success with the mylar,it leaves a perfectly flat mirror like surface giving the wood grain depth.If you get it right you get a perfect reflection and no faring is needed.Maybe as you said Eric a thinner material maybe better like 6 mil plastic but this may cause waves in the surface.
    Im gonna try puddling the epoxy in the middle then lay the mylar over pushing the goo to the sides.Very fussy stuff but if you get it right boy does it look good.
     
  4. PAR
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    PAR Yacht Designer/Builder

    I've been using the Mylar (and other sheet goods) trick to smooth out epoxy for many years and it works well.

    Your problem sounds like out gassing and this isn't a problem caused by the Mylar. The wood (or surface) must be sealed, before this technique can be employed. If it's wood, it needs a minimum of two coats of epoxy and these should be applied in a specific way to eliminate out gassing.

    On raw wood (only), the first coat of epoxy needs to be applied in a thin coat (not diluted, just thin). More importantly it needs to be mashed into the surface, then scraped off after it's had a chance to soak in. This is all done with the squeegee, putty knife or plastic applicator, used to apply the straight goo (a brush can't do this). Basically you smear it on, mashing it into the surface as you go, wait a few minutes to let it soak in good, then scrape the surface to remove any pools of epoxy. The surface should appear as though it's gotten wet, but not shinny. If it's shinny, you have a pool of epoxy on the surface, which trapped air bubbles will try to travel though, eventually popping on the surface if they can. If there isn't any pools of epoxy on the surface, then the gases trying to escape the wood, will reach the surface and just vent to the air, rather then try to rise up though a viscous liquid that's also about to cure.

    You can use the hot on hot method too, which permits faster film thickness building, but requires more "tending".

    Only once the surface is sealed, can you consider the Mylar smoothing technique. Other wise trapped air trying to escape, will just get stuck under the Mylar and you'll get a fisheye looking thing or a bubbles.

    The Mylar trick is a finishing technique and shouldn't be used until you're ready for that stage. Initial epoxy coats are to seal the substrate and build film thickness, which is a different part of the process.
     
  5. Tungsten
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    Tungsten Senior Member

    The surface has been sealed already with glass and epoxy
     
  6. tom28571
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    tom28571 Senior Member

    Paul & Eric,

    You are missing something here. There was a member of the forum that posted a project a few years ago that did exactly what tungsten is talking about and he used 20 or 30mil poly to smooth full side areas of epoxy on a semi submersible. I saw the photos and they looked beautiful with no obvious bubbles. I tried the same thing on the bottom of a runabout and had the same result that tungsten did. I was unable to install the film without getting bubbles, lots of little ones. It was not outgassing, just a product of the installation. Shoving bubbles toward the edges was not successful on such a large area. From 6 or 8 feet, it looked really great, like a mirror. Maybe the bubbles just did not show in the photos I saw and maybe the OP did not mind them, or maybe he was able to solve the problem.

    Like you guys, I have done this in small area repairs with fine results but the large area of a whole half bottom were another level of problem. Filling the bubbles was not easy either with needing to poke a toothpick dipped in epoxy into each one. Trying to fill them with thin or thickened epoxy and a squeegee was not very successful either.

    Thinner plastic might help with the bubble problem but would not leave a fair and level surface over a large area like the thicker material. I know this has been done as far back as the 1960s with gel coat on whole boats but the photos I remember showed several people at work doing the job. Maybe they were able to feed the film and chase the bubbles much better with more hands.

    Anyway, I was unwilling to chase the problem further and went back to the usual methods.
     
  7. PAR
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    PAR Yacht Designer/Builder

    One thing I have noticed (with this technique), is static can attract all sorts of light particulates to the film and this needs to be addressed, as (or before) the film goes down. I've also found you do have to roll the film slowly into the goo. Lastly I usually apply the film with a squeegee, just behind, so as the film unwinds from the roll, it's pushed into the goo. All attempts to "maintain contact" are made to prevent bubbles. I also have attempted too big an area with film and had this should be avoided too. I like 18" and 24" rolls and work the "developed" angles. 12" rolls means more effort, but less chance of trapping air.
     
  8. Tungsten
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    Tungsten Senior Member

    Like you metioned Par i've tried the square edge pushing as it un rolls,little bubbles still find there way in.I've only had sucsess on very small areas by just gooping in lots of goo.The problem is getting rid of them once there there.You dont want to push them to the edge as epoxy goes with it.What about heat?I havent tried that yet.
    I'm also using new film with the protective coating that i remove.
    maybe a thinner mylar?
     
  9. Eric Sponberg
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    Eric Sponberg Senior Member

    All you folks have more experience with this than me, so I am learning something here. Sounds like PAR has been there, done that, and perfected the techniques. I would bet that air and/or impurities are coming from underneath or at the interface of the wet resin as it is touched by the Mylar sheet.

    Question, just to trouble-shoot this for a minute: Tungsten, what is your mixing technique with the resin? You mix the two parts and then stir it--do you let the resin sit for a few minutes before using it? How are you brushing it on or applying it to the surface? If your whole process of mixing and applying happens relatively quickly, you are going to introduce air into the resin that wants to get out. There are the large air bubble certainly, but there are also lots of tiny minute little bubbles that get created by mixing and brushing. So the solution might be to slow the mixing and application down, perhaps letting the resin sit uncovered for some minutes before rolling the Mylar on--give the resin a chance to let the air escape. Just a thought--trying to lend an extra mind to the problem.

    And I am learning something--thanks!

    Eric
     
  10. PAR
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    PAR Yacht Designer/Builder

    Eric has touched on something that I didn't mention - mixing. This is the way most air gets into the resin. Most mix in a relatively confined pot (mixing cup) and stir, which introduces lots of air, especially if you mix aggressively.

    I use a folding technique to mix resin (and fillers), which dramatically decreases bubbles. I blatantly stole this technique from a printing press operator many years ago. He mixed inks this way and insisted it was the only way for a complete mix. Instead of a mixing cup, I use a large (12"x18" or bigger), flat bottomed, shallow tray. I pour both resin and hardener in and tilt the tray slightly so it pools at one end. Then with a plastic applicator, I drag the goo up hill in the tray, which causes it to flow back down hill as I do. The action is deliberately slow. The folding action is more useful with fillers, but I do the same with straight resin. I scrape the bottom of the mixing tray and fold this accumulated pile of goo onto what remains in the tray.

    If I incorporate fillers, I'll switch from the big, shallow mixing tray to a piece of plywood (precoated), so I can spread out the goo into a larger and thinner mix. I again drag out the goo (this time flat) into a thin pool. I scrape this up and fold it over onto the remaining goo. It's a back and forth motion, folding and moving the goo, back and forth and a bit hard to describe. It's effective and if you move slowly, doesn't introduce too many bubbles.

    Lastly, yes waiting will permit bubbles to rise and pop on the surface, especially if spread out in a thin pool (why I use the big mixing tray or a plywood hock). I usually let the goo sit a bit, if it's going on an area to get a bright finish. If it's really spread out (less than an 1/8" think, preferably thinner) the bubbles will quickly rise and release themselves from the goo. If in a mixing cup, it can take a long time for a bubble to rise from the bottom, through the relatively viscous liquid and pop on the surface. If using a brush or roller to apply goo, you'll also introduce bubbles, so I use a squeegee, plastic applicator or putty knife (I know you do too, Tom), which decreases the bubble thingie.
     
  11. tom28571
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    tom28571 Senior Member

    In my aborted attempts to get what I would call satisfactory results on large areas, I am pretty sure where the bubbles come from and you guys have mentioned both. I do not think any of them came from the wood base. The majority are formed when the plastic makes contact with the surface and they are trapped underneath. A lesser amount likely come from the epoxy interior from mixing. On small areas, the surface problem can be beaten by applying an excess of epoxy so that the surface bubbles are squeezed out on the edges. That does not work on large areas because an excess of epoxy would mean several times more than needed for the job. Since I have no problem with entrained bubbles on small jobs, I don't think that is a major source on larger jobs either.

    Thinking back on the photos I saw in the 60s, I think this is how they did it with gel coat:
    They worked on vertical side surfaces from bottom to top. Some poured gelcoat from the top into the contact area of rolled film and hull. Others squeegeed upwards from bottom to top, keeping a (relatively) large amount of excess material just ahead of the squeegee as film roll was moved upward by other workers. This made almost perfect contact with the film before it was pressed against the hull. In this way, bubbles were not allowed to form and those that did could escape before being trapped.

    This kind of duplicates how I work with small repair areas where I can control everything. I think you can get a smooth surface with thinner film but doubt that a large surface can be made fair with the thinner stuff.
     
  12. Tungsten
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    Tungsten Senior Member

    The fish eyes or bubbles that i'm getting i think are coming from not having the epoxy spread evenly.I've done a small test area and by spreading the goo with a notched trowel then rolling the mylar with little pressure seams to have solved my problem.I also tried dropping a puddle in the center then laying the mylar,with gental pressure the goo moves away and flattens out.But if you push too hard the mylar bounces back creating a bubble.

    Now i'll have to tilt my boat so the sides are horizontal so the goo doesnt run away.
     

  13. SamSam
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    SamSam Senior Member

    I think I first saw the technique in the Duckworks Magazine site. This guy is doing a similar thing,

    [​IMG]

    http://www.eyeinhand.com/Marginalia/2010/06/01/turning-for-home/

    but the end result has sort of wavy reflections of light. In the original article, the builder used a heavier mylar on a developable sheet build that left the sides high gloss shiny with perfectly fair straight light reflections.
     
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