Micro-balloon Paint?


Curious about Wooden Canoes
I'm in the process of re-canvasing my 15' custom built. In the past I've played with a variety of sizing techniques.

1. Clay-based filler + marine paint
2. Searing canvas with torch + polyurethane + porch paint
3. Polyurethane + Porch Paint

All of these techniques have held up well, although the filler adds considerable weight. Canvas has been acceptably smooth when I scorch it but don't fill. I've been reading through recent posts trying to figure out what to do this time and seen the good suggestion of using micro-balloons as a filler. Has anyone tried adding micro-balloons directly to paint or polyurethane as a sizer? Results?
I just ordered microballoons and am planning on putting it in the filler or the primer. I'm thinkin' in the primer more than the filler, but haven't done it yet. Seems like it would work.
Something to keep in mind about microballoons is that unlike microspheres, the balloons actually are hollow. When you sand a microballoon fill, you will be cutting some of the balloons near the surface open, producing tiny, air-filled craters on the surface. Some topcoats won't bridge these craters, leaving tiny pinholes. Others may bridge the gaps, but aren't strong enough. If the surface heats up in the sun, the air inside the craters can expand and rupture the overcoat - leaving similar pinholes behind as they do.

For these reasons, most of the resin manufacturers who also supply microballoons suggest doing your fill, sanding it smooth and then top-coating it with plain epoxy (which is thick enough and strong enough to seal the craters). That epoxy coat is then finish sanded smooth and painted.

While the tiny pits left behind from balloons aren't a huge problem, they can be annoying and may tend to spoil a really nice paint job. I don't think there is any way I would ever try adding them directly to paint, as it seems to be just asking for trouble. Adding them to primer or filler might be a different story, but it might be worth doing some testing and leaving the samples out to heat up in the sun for a couple of days to see if your top coats are strong enough to keep these little spots sealed and smooth.

Most common fillers are pretty stable, but the two that can be risky at times are microballoons and aluminum flake powder. The aliminum doesn't have the air-expansion or crater problem that balloons can have, but the stuff starts to oxidize just as soon as you're done sanding it. Rather than painting over fresh-sanded aluminum, you're actually trying to paint over a thin film of aluminum oxide and it can result in similar tiny flaws spread out on your paint job. I had one boat like that and managed to convince myself that it was no big deal for the better part of a year before I finally got out the sander and ground the paint and the aluminum powdered layers off and repainted it properly.

Thanks for the suggestions about the micro-balloons. I guess one question that comes up is what is actually providing the waterproofing of the canvas. I always assumed it is really the paint or polyurethane, not the filler. Is that a false assumption?

Given that the mirco-spheres are solid, is there a significant difference in weight between those and the balloons?

Finally, about the epoxy. Other than cost, my primary concern with using it is that it will make future re-canvasings a nightmare; I worry that the spoxy will seep through the canvas and adhere it to the hull. Anyone have experience to the contrary?
The actual water barrier would pretty much be the paint (or whatever vehicle the particles are suspended in) although there is some truth to the idea that filling it with tiny chunks of waterproof materials will increase its ability to resist water penetration. For example, this is how epoxy barrier coats work for blister repair on fiberglass boats. The epoxy coating they use (which is already very water-resistant by itself) is made even more so by addition of aluminum flake powder. Any water trying to get in has to not only penetrate the epoxy, but it also literally has to zig-zag around the tiny aluminum flakes. Seems kind of crudely mechanical when you think about it, but the results are measurable.

Microballoons are most likely lighter than a similar volume of microspheres, but I don't know by how much.

You are correct about epoxy seeping into canvas. Cotton absorbs epoxy like crazy, and lots of it. The result would be extremely heavy and very difficult to remove. Not a good idea. In general when it comes to fabric coverings for rib and plank canoes, I think you will find it very difficult to beat traditional canvas and filler or possibly Dacron if you are willing to live with the limits that it presents in order to save weight. Folks have been trying to reinvent the wheel on these things for decades, but I think there is more behind the reason that most of the top builders and restorers still use traditional filler than just nostalgia.
Excellent info. Thanks so much. I guess that means I simply need to weigh the advantages of a slightly smoother surface over and the added weight that comes with it (filler) versus the weight, cost, & times savings to had through a coat of poly followed by paint. No filler.
Probably should also factor general durability into that equation as well. Despite being basically modified paint, good filler is pretty tough stuff - in kind of a semi-hard, sem-flexible way. That's also one more way that microballoons used to replace some of the other filling components would tend to come out on the short end of the stick. The reasons that they are popular for jobs like making fairing putty are light weight and very easy sandability. They will bring both of those characteristics to any party that you invite them to, so replacing some of the harder, heavier materials with balloons is going to reduce the abrasion resistance of the filler. Suitability then depends on what you plan to run over - though it's often the things you didn't plan to run over that get you.

The first time I ever removed canvas from a canoe, I took a chunk of it that was in pretty nice shape and decided to see just how tough it was. I grabbed a really big nail and dragged it across the surface of the scrap, pressing quite hard. It scraped through the paint and into the filler, but that was all it did. The stuff was a lot tougher than I thought it was and it gave me new respect for the durability of filled canvas. At that point in canoe-history more whitewater miles had been logged on wood/canvas boats than on fiberglass or plastic canoes and though there was no way I was going to take my wooden canoe down through a rock garden, I certainly had a better understanding that it could be done.
I hear what you're saying, and it all makes a lot of sense on paper. The main reason for recanvasing is that I have run my boat through a whole lot of rock gardens and the skin paid the price (as well as a couple of ribs and a plank or two). I'm always super impressed by the durability of the canvas.

However, what I keep thinking about are the canoes I used at summer camp as a kid. They were all covered with an unfilled canvas and they took every bit as much of a beating as did my current boat with filled canvas. Given that it's always seemed like the main advantage of filling is a smoother finish, not a tougher one. But it's good food for thought.