Micropores on surface

Discussion in 'Materials' started by Yellowjacket, Aug 19, 2012.

  1. Yellowjacket
    Joined: May 2009
    Posts: 662
    Likes: 112, Points: 43, Legacy Rep: 447
    Location: Landlocked...

    Yellowjacket Senior Member

    I did the mylar over epoxy technique and squeezed out all of the bubbles but when the epoxy set and I removed the mylar I had some places where very tiny bubbles came to the surface and left very small surface imperfections. I found that i could fill these with a surface coat of epoxy, but when that set up the pinholes came back, the epoxy was repelled out of the holes, I know I had them filled when I coated it. I worked it with the brush and they came back as the resin cured.

    I'm assuming that there is an issue with blush or for some reason the epoxy is getting repelled from the bottom of the tiny pock mark.

    I am thinking that I will have clean any blush, and then sand down to the bottom of the pin hole pockets to clear this up.

    Par, Any thoughts????
     
  2. gonzo
    Joined: Aug 2002
    Posts: 14,758
    Likes: 759, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    There is air left in the pinholes. When the epoxy cures it heats up, the air expands and pushes the epoxy out. If you heat the surface before laying the epoxy, the air will contract when it cools and suck the resin in.
     
  3. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 477, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    It could be air, but it also could be some form of contamination too. Clean and scuff up, then as Gonzo says, heat the surface, say to 100 degrees with a hair drier or heat gun, then apply epoxy after letting it cool for a couple of minutes (real important).

    The Mylar technique should only be used over sealed wood or non-porous surfaces, other wise out gassing can occur. Also when using this technique, let the epoxy stand for a while before putting the Mylar on, so any bubbles made during mixing or application, can rise to the surface and pop. In fact, I'll run a propane torch over the surface to insure I've popped all the bubbles, then down goes the Mylar. This trick makes a pretty sweet surface though doesn't it Yellowjacket?
     
  4. Yellowjacket
    Joined: May 2009
    Posts: 662
    Likes: 112, Points: 43, Legacy Rep: 447
    Location: Landlocked...

    Yellowjacket Senior Member

    Par,

    It sure does make a great surface, and I did a seal coat prior to applying the cloth. The night before I coated the surface with a "no blush" epoxy and started the work in the morning. It was almost tack free when I laid down the glass. It was actually warmer in the garage than outside when we started, and then opened the doors since it was pretty warm. The ambient temp in the garage was close to 80 F, and we opened the doors and it was about 75 outside. It was pretty constant in terms of temperature as we worked. The bubbles weren't from outgassing. I'm pretty sure they were from too much working of the roller (mental note... DON'T use a roller if your are going to use mylar unless you do it really slowly). The areas that I used a brush on didn't bubble at all.

    I worked a rubber roller over the mylar pretty hard to get most of the bubbles out from under the mylar. I could see that they were there and in most cases the mylar was only 8 inches wide, so I rolled them to the edges and out of the edges of that particular mylar sheet.

    When I took the mylar off the finish was perfect, but the tiny bubbles were very visible. At first look it appeared they were deep in the finish, (I came close to ripping it all off and starting over but convinced myself it wouldn't be that bad to live with it. In some places I could feel a tiny amount of roughness at the surface so I decided to try to fill the ones I could feel, but basically all of them were at the surface, and coating with a brush filled them and they disappeared, but now the surface has tiny "dimples" literally the size of a pin prick, but if you look straight down on the surface it is filled enough that it is clear, you can't see the imperfection. I'd call the surface "fisheyed", but usually fish eyes are a lot bigger. These are very tiny. I'm assuming that if I sand it down to the original surface I will be fine. This is supposed to eliminate sanding, but at this point, at least if I can salvage it and make it look proper I'll take it.

    Thanks for the tip on heating the surface, I will do that on the upper surfaces. Also plan on using a brush to apply the epoxy.

    We (I say we because it was a team effort, the wife, and kid were all in on it too), were concerned about setup time (and probably shouldn't have been) and we put the mylar on right away. The boat is 13 ft long and 60 inches of bottom surface at the transom, and I had to do it all in one shot. If I had been doing smaller pieces it would have been easier, but in this case the glass had to do the whole bottom and up the transom with no seams, so I was concerned we would have problems with it setting up. In hindsight, the areas that sat longer before I got the mylar on didn't bubble as much.

    When I saw the bubbling under the mylar I ended up rolling out most of the time while the wife applied epoxy. I was rolling hard and long, and the other small pieces I've done with the technique weren't nearly as hard to roll out, and came out a lot better.

    I will let it sit for a bit next time. Also, will a heat gun pop the bubbles as opposed to a propane torch? I've heard of folks waving a heat gun over the applied goo to do the same thing.

    The plan now is to let it set up until at least tomorrow night, then clean the blush off, and sand most of what I brushed on last night and hopefully get to the bottom of this (literally, the bottom of the imperfections). I won't go as far as the cloth, it looks like just a bit of sanding will take it down enough to be a smooth surface. I may do a final light coat of epoxy over the sanded surface, I should know more when I sand it.

    I also have a bit of visible weave, but I think that may also be related to using the roller, the other pieces I've done with a brush were perfectly transparent.

    I did one small (20" x 12") area using mylar directly over stained wood and it came out excellent (beginners luck maybe) That was over plywood and the temperature was very hot and cooling off when I did it and didn't have any issues with gassing. Actually that is the clearest and best piece I've done so far.
     
  5. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 477, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    A heat gun can do it, but it takes a lot longer and on a large surface you usually don't have the time to spare, so I use a torch. I use MAPP gas too, instead of propane, which is hotter. Just keep it moving and watch the work. You'll see the bubbles popping.

    If using a roller, it's best to tip off. You can do this with the roller. Roll out the surface, with an eye towards getting a uniform coating thickness, then place a finger on the roller and drag it slowly and lightly back through the area, which will pop most of the bubbles. In fact, if you're methodical about it, you can do this with each pass of the roller: roll it up, lock the roller, drag it back, move over and start again. You can tip off with a foam brush too, which works as well, if not a little better.

    The Mylar technique is for a finial finish coat, so the weave needs to be filled previously to this technique (as you now know). The Mylar trick is really great on un-sheathed pieces, like table tops, counters, shelves, etc. I did a bathroom countertop for a friend and used this trick. Real pretty yellow cedar, edge glued, two coats of goo to seal and level the surface, a light sanding, with the grain then a top coat with the Mylar trick. He was amazed and called it machine made. We cut the holes for the two sinks, then varnished it up. Had he not seen the process, he wouldn't have believed me, that it was hand applied. Interestingly enough, he didn't see the Mylar part, which I was careful to conceal. You have to keep them guessing . . .
     
  6. Yellowjacket
    Joined: May 2009
    Posts: 662
    Likes: 112, Points: 43, Legacy Rep: 447
    Location: Landlocked...

    Yellowjacket Senior Member

    I agree that it is best done as a finish coat. If doing it again, over cloth I'd think that one could wet out the cloth and squeegee it out as in a conventional finish and then do the Mylar over that after it set up, but hadn't cured, to wet out the weave, and it would work great and be easy as pie.

    I had seen the duckworks stuff on the technique and on that page they claimed to have actually "compressed" the weave doing this over cloth. After doing it I'm more than a bit skeptical about that claim. Seemed like no matter how much goo I put down, I didn't get much more out the sides than if I put less down. I had a couple of small bubbles and there was a lot (like .020 or .030") before you got to the cloth. My thought is that it gets leveled by the Mylar, but once it is level it doesn't compress that much more. I was using the 30 mil Mylar so there is a limit as to how much flexibility there is in that material. If you did a squeegee job and hot coated it, you might use less goo in the first place.
     
  7. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 477, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Your observations are correct Yellow, the Mylar trick is a finishing technique, and isn't going to be very useful without a vacuum pump, moving resin around and then you'll need a bleeder fabric, defeating the whole thing.

    Wetouts need only so much resin and hand applications can only get so much in resin/fiber ratios. You can very marginally improve these figures, but not enough to warrant the bother. Resin isn't going to compress much, if at all anyway, it's a viscous liquid, so ignore the Duckworks fodder and stick with what you've learned, which is clearly more then the writer over at Duckworks.
     
  8. rwatson
    Joined: Aug 2007
    Posts: 5,862
    Likes: 296, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    No, it can be a fact. What you and Par say is quite correct on your basic ... say 6 oz woven cloth, but some fabric, that may tend to sit up a bit, will get compressed. This is most notable if you use a bit of matt as well as cloth under the Mylar, for example.

    I found that once you roll the Mylar, that normal atmospheric pressure keeps the Mylar pressed down more than the surrounding fabric. Air cant sneek back in under the Mylar as the epoxy is quite thick. Its the same effect as wetting a piece of leather in the school lab, and pressing it down on a glass desktop.
     
  9. SamSam
    Joined: Feb 2005
    Posts: 3,900
    Likes: 197, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    When doing sheetrock, small bubbles can form that leave small pits when sanded. To fill them on a second coat usually involves application in one direction and then working it from a few more directions, 180 degrees, 90 degrees, etc. to work the trapped air out of the pit. The same with the screw dimples to begin with. Not introducing air into the materials when originally mixing helps a lot.
     

  10. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 477, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    The act of mixing introduces a lot of air, in epoxy, regardless of mixing methods. I use a printing press ink mixing technique that is the least likely air producer, but there' still going to be air, as the mixing tool dips in and out of the goo.

    If you use a roller, you'll get lots more bubbles, but tipping off will remove a high percentage of these. If you run a torch over the surface immediately after application, you'll get the rest of them. Again this technique is purely for finishing, not well suited to interlaminate bonding. That's what bagging is for.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.