By Waid Reynolds

No, this isn't about looking in the lost and found for that foam core you can't find. Recently, I wrote up some instructions on lost foam construction for a friend who wanted to build a lightweight fiberglass fuse for a speed 400 pylon racer. I thought the information might be useful as a guide to others who want to try this technique. I have built several fuselages this way. It can produce a very nice end product. Without further ado...

Use "Satin Weave" glass cloth for the main layer(s). Satin weave conforms well to compound curves. Usually it is only available in heavier weights like 5 ounce. For a very light fuse you will might need only 1 layer over all with another layer of lighter glass in high stress areas. You may also want to add carbon fiber and/or Kevlar in high stress areas.

Wear protective gloves when handling epoxy. Work in a well ventilated area.

The process goes something like this:

1. Carve the foam core to the desired shape.

2. Mount it on a fixture with a nail at each end to hold the foam core off the table. You want to be able to rotate the core like a chicken on a rotisserie so that you can work on all sides of it.

3. If you use other than white foam, cover the core with packing tape. White foam dissolves easily with white gas, so you don't need the packing tape. Blue foam is easier to carve, but leaves a lot of residue when you dissolve it. The packing tape allows you to remove all of the residue by merely pulling out the packing tape from the inside of the fuse.

4. Cut glass cloth in overlapping strips. Make the strips as wide as will conform to the shape of the core. In some areas they my be quite narrow, and it others quite wide.

5. Lightly spray the core with 3M #77 spray fix. Let dry for at least 10 minutes.

6. Lay out the glass strips for the FIRST layer on a newspaper and very lightly spray one side with 3M #77 spray fix. Let dry for at least 5 minutes. #77 will remain sticky for several days, so you don't need to do the fuse all at once if time doesn't permit.

7. Apply a layer of glass to the core. You can cover it completely, or work on just a portion at a time.

8. Mix your epoxy per the manufacturer's recommendations.

9. Brush epoxy into the glass, thoroughly saturating it.

10. If another layer of glass is to be applied, lay it on now. Work it into the epoxy. Brush on more epoxy if needed. Continue adding more layers until done.

11. Use toilet paper or paper towels to absorb as much of the epoxy as possible. I always double up the toilet paper at first when there is still lots of epoxy on the glass. Lay the paper on a portion of the fuse. Press the paper onto the glass so that it absorbs the epoxy. Gently pull off the paper, being careful not to pull up glass cloth with it. Continue this process until no more epoxy can be picked up by the paper. This usually requires from 3 to 6 applications.

12. Let the epoxy harden for at least 24 hours if no more glass is to be applied. If more glass is needed, apply it before 24 hours to get the best bond. You may want to rough up the surface with sandpaper first if it is fairly hard, or if the epoxy has set for more than 24 hours.

1. After the epoxy has hardened, sand off the high spots and rough up the surface all over with medium grid sandpaper.

2. Mix up a blend of epoxy and microballoons that is just liquid enough to be brushed on, but not so fluid as to be runny. You want plenty of microballoons so that it will be easy to sand. Other fillers that will stick to epoxy can be used. Just be sure they are light in weight. A relatively fast curing resin, say 20 minutes, is preferred so that it will not drip off of the fuse before taking a set.

3. Brush the filler all over the fuse. Let it harden until it can be sanded. Don't let it harden any longer than necessary, because that just makes it more difficult to sand.

4. Using sandpapers of decreasing roughness, sand the mixture all off, except what is left in the hollows. One application may be enough to get a smooth surface, but you may have to touch up a few areas. Wet sanding works best.

5. Cut out the canopy or access hatch. I just lay the fuse on its side and cut out the canopy on my band saw (flat fuse sides facilitate this). This exposes the foam so that you can dissolve it with white gas. Pull out the packing tape if the fuse was lined with it.

6. Spray paint with white or light gray (depending on the finish color you want) Krylon sandable primer or something similar. Wet sand almost all of it off. Repeat this process until you are happy with the surface smoothness.

7. Spray paint the fuse with one or two coats of Krylon fast dry enamel.