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Bondo, Foam, and Failure

Posted by qwindelzorf on April 8, 2011

I'm RedPeasant, and I'm the other half of the crazy Portal Sentry team. I'm going to be putting together a few posts for the build log. The first few will just be catching up to where we currently are with what is mostly a long string of failure.

So, we had this crazy idea to make one of the sentry robots from Portal and milled a bunch of styrofoam to shape. Then we started trying to finish the things. My original plan had been to mill the foam, get it cleaned up, then lay up fiberglass over it for strength. Well, There were a couple problems. I tried to bondo the foam to cover low spots in the foam leftover from milling and stacking the layers. That's when I discovered an obnoxious truth: Everything Eats Foam.


Bondo eats it, aerosol sprays eat it (we already knew that one), and the fiberglass resin would probably end up eating it too. You can see it a little bit in the picture that the bondo spots are a little lower than the surrounding foam. This really becomes a problem when you're trying to use the bondo to fill low spots. The solution here was to give the whole thing a couple good coats of Kilz to seal the foam against everything. After that, everything went ok for a while. We ended up with one pod and half of the body:


At this point, the pod doesn't have any of the geometry on the other side; it's just a solid block straight down from the edges. We made a hot wire cutter and gave that a try. The first power supply that we tried died when we fired it up, so we found something at the local Home Depot that would serve our purposes:


Well, it worked pretty well for a couple test cuts, so we started on the pod. Here is the result:


That's the hot wire rig with the pod. Note the nice big gash in the foam where the hot wire sliced it when it snapped. Whoops. The hot wire cutter is only the first of many things that would go seriously wrong with this project.

I had been using a hacksaw blade to carve foam up until this point, and we decided that a bow saw blade would be long enough to cut the curve we needed. It was a pretty rough cut, but it worked:


I used a little sanding and bondo to fix the edge of the seam, and it ended up looking pretty good. Then we had problem number 2. For a while, I had been noticing little bubbles under the paint. Upon closer examination, they were actually under the thin layer of bondo, and the problem was getting worse. My best guess is that the bondo wasn't adhering well to the Kilz/foam and was coming up when I sanded it. There wasn't a good way to fix this, and the surface finish would quickly deteriorate if we just kept going, so we decided to give something else a try.