It’s customary to heat-seal EVA foam before applying primer and beginning to paint. But what is heat-sealing, and why should you do it?
To explain this, we need to talk about open-cell and closed-cell foams. The tiny cells that the foam materials are made out of can be open or closed. You can think of these cells as little microscopic footballs. For example, an old football with holes in it would let air and liquids through. The football has openings. If this happens in a cell, we call this an open cell. Similarly, a football that isn’t broken and has no holes, would not let air and liquid through. The football is closed off from those things. We’d call that a closed cell. As a general rule of thumb, if at least half the cells are open as opposed to closed, a material is referred to as open-celled. Otherwise, it’s considered a closed-cell material.
Kitchen sponges are a great example of an object made out of an open cell foam. There isn’t much there, and all of the holes and nooks and crannies are perfect hidey-holes for liquids such as water. Because the vast majority of the cells are open, you can put water in on one side of the sponge, and squish it out from the other side. If you were to put a lot of paint on top of a sponge, it’d almost certainly start coming out the the other end very rapidly.
EVA foam is considered a closed-cell foam. On a microscopic level, the material is made up of numerous tiny closed cells. These closed cells do not let air or liquids pass. As such, you could put water on top of EVA foam, and it would not seep in through the material to the extent it would in an open foam material.
There is something else to consider, however, and that is that even closed-cell materials may still be porous, and may still have some open cells. (This is more true for the more cheaply produced EVA foam with lower densities, which have larger cells with more air in them; and less pronounced on higher-density, higher-quality foam.)
Consider the following example of EVA foam:
Notice that there are tiny gaps between the cells (which, for the purpose of this explanation, I have exaggerated). If you were to apply paint or primer to this foam as-is, then the liquid would inevitably crawl into these gaps between the cells (and through any cell that may accidentally be open, which can happen as a consequence of the production process). Because your paint would seep through, even in small amounts, you’d need to apply more to be able to get a sufficiently smooth application on the top layer:
We can work around this issue with a process called heat-sealing. By applying heat to the surface of the foam, we can essentially “melt” the top layer of cells. This causes the cells to deform slightly and sink into the bottom layers through any gaps that may exist, or any open cells that may happen to be there:
You literally seal the gaps with heat, thus the name. Because this process closes the same gaps in the foam that the paint in our previous example was able to seep through, applying paint after heat-sealing takes far less material:
That’s a remarkable difference! And if you’re using quality (read: pricey) paints, the resulting cost savings can be equally significant.
So how does one heat-seal foam? There are two commonly used methods. The first is using a heat gun, and the second is a butane torch. The process is similar for both, but we’ll focus on the heat gun here, as it is the safer of the two choices.
If you haven’t used a heat gun before, it’s essentially a super-powered hair dryer. While hair-dryers max out at temperatures around 60 degrees Celsius, heat guns can reach 600 degrees Celsius with ease. A hair dryer wouldn’t work to seal EVA foam as it requires the much higher temperatures that heat guns are able to achieve.
To use a heat gun to seal EVA foam, simply let it heat for a few seconds, and then blow hot air onto your foam from a distance of about 20 centimeters. It’s actually possible to burn your foam by keeping it focused on a single area for too long, so keep it moving! I prefer to move at a rate of approximately 10 centimeters per second. As long as the air is hot enough, it really doesn’t take much. Once the first pass is finished, you may do a second pass if you feel it’s necessary. Congratulations! You’ve just sealed your EVA foam!
Safety Tip: Once you’re done, it’s important remember that the business end of your heat gun will remain hot for quite a while. If your heat gun came with a storage case, don’t put it back in immediately. Place your heat gun somewhere where it can cool off and not be knocked over. Keep it out of reach of kids and pets. Remember not to put the business end of your heat gun on or near materials that are flammable or may deform from the heat, or otherwise result in a hazardous condition, such as near gas or power cables. If you have a fan, consider using it to help cool off your heat gun more rapidly.
I hope this article was informative and helpful!