Fairing Model: Tool Finishing
Dateline: 06/09/99Additional Support Required
With the tool surface roughed in, all that remains is to apply a surface coat to make it nice and smooth. Well, almost all.
Although the tool looks very level, there's still a bit of sag left. It's not really visible, but we can hear the tool creak as it rotates. After several rotations, some of the foam joints begin to separate a bit.
At this point, it's time to resurrect the roller supports. The rollers are simply two wheels on a metal frame, placed below the central bulkhead.
The rollers are bolted through a slot so their height can be adjusted. The tool is lifted slightly to make it level, but the weight is enough to push the rollers down, no matter how much we torque the bolts. Once we get the height set properly, we have to use aluminum posts to keep the rollers from slipping down.
The tool is now ready for finishing. There's one last problem, though: the rollers will interfere with the surfacing and the layup. Both the surface coat and layup will be crushed as the tool is rotated. At the moment, our plan is to continue as is and then repair the part after we remove it from the tool.
Bondo
The tool surface has been roughed-in with a hotwire cutter, but it's still not smooth enough for a layup tool. There are some dips where the hotwire went too deep, and some voids in the expandable foam joints. (There had been some high spots, but we sanded those down earlier.)
These lows spots need to be filled in. The entire tool also needs a thin surface coat, because the porous foam will absorb resin and bond to the layup.
The classic material for this finishing work is Bondo, or a similar filled polyester. A local auto supply shop sells a toughened version in five gallon pails.
To get a smooth surface, we built a wooden spreader which extended between the bulkheads (which had been sized to the nominal OD of the tool). The spreader was covered with Teflon tape, and ordinary cabinet handles made good grips.
Spreading the filler compound wasn't very easy. It was thick, and even with the Teflon tape it tended to stick to the spreader. Furthermore, the large spreader couldn't work the material into the small voids , and we had to use plastic squeegees over much of the tool.
Sanding
Because the filler was so difficult to spread, it didn't go down very smoothly. In some cases, the ridges were close to an eight of an inch above the nominal surface.
Being a toughened filler, it also wasn't easy to sand. To maintain the proper surface height, we first tried using the large spreader as a sanding block.
Sanding by hand turned out to be much too slow. To speed things up, we switched to an air-powered sander (like a giant palm sander). This was faster, but still rather slow. Also, the sander didn't extend between the bulkheads, so it was harder to maintain the proper surface contour.
Finally, in the really tough areas, we had to use an electric belt sander. This took off the material very quickly, but it was almost impossible to maintain a good contour.
Joint Compound
It became obvious to us that covering the entire tool with Bondo would take too long, both in spreading and sanding time. Also, the pesky sag had come back to haunt us once again. As we rotated the tool, cracks started to appear in the surface coat.
After a bit of brainstorming, we came up with an approach that solved both the sag and the finishing problems. To speed up finishing, we decided to use joint (or drywall) compound. One of our technicians had just refinished his house, so he had plenty of practice. Plus, joint compound is a lot cheaper than Bondo.
Of course, it's nowhere near as strong, so the cracking problem would be even worse. But if the tool doesn't rotate, then the surface can't crack. The solution seemed obvious once we thought of it: We would finish the upper half of the tool, layup one half of the part, rotate the tool, then finish and layup the other half. Finally, we would bond or bolt the two halves together.
Plastering went very quickly. One person dumped the compound on the tool, and used a small drywall blade to work compound into the voids. Our home remodeller followed with a larger blade (the largest we could find), smoothing out the surface.
After the first coat dried overnight, we sanded it with the large Bondo spreader. After another quick coat and a final sanding, the tool was finally ready for the layup.
