This is presented as food for thought, not
as the only way to do it. This idea was developed by Cory Bird, a very bright
manufacturing engineer at Scaled. Cory is in the finishing stages of his exquisite
original design and came up with this idea while working through the contouring stages on
his airplane. I recently refinished my wood core/carbon composite prop and I used Cory's
idea and I liked it! It worked great! Here it is.
The idea came when Cory compared the weight of a gallon of epoxy with a gallon of Featherfill. if you have not done this, do it, it will open your eyes! Even taking into account the evaporation of solvents in the Featherfill, there is a huge difference. Anyway, this is a process that starts when you have your airplane (or parts of airplane) structurally complete, in bare glass, and are ready to begin contouring. Sand the glass as usual, you are not looking for a structural bond such as you would need in a glass-to-glass bond, you just need to scratch the cured epoxy. Use at least 40 grit, 36 grit is better. Sand hard in one direction 10 strokes. Then sand hard at 900 to the first sanding in the same area, 10 strokes. This is not a hard and fast rule, it is just a rule of thumb so that you can begin to see the kind of surface preparation you need prior to applying dry micro.
Before applying the dry micro, paint the area with pure epoxy. Wipe as much of this epoxy off as you can with a clean paper towel. This is the .glue" that will bond the dry micro to the cured substrate (skin). A good idea here is to only try to do a small area at a time, say a square foot or two. Mix up a batch of dry micro - the consistency of cake icing works well. Some people try to mix it so dry that it is almost impossible to apply. I don't agree with that. The gram or two of weight you might save per 8 ounce cup is simply not worth the enormous effort. Spread the micro (just like peanut butter) using a squeegee. If it rolls up behind the squeegee, it is a little dry but you can fix that with peel ply. Squeegee through the peel ply to get the micro even and where you want it. Once the whole surface is micro'd, allow it to cure.
Contour sanding should be done using a long sanding block. In the case of a wing, 3 or even 4 feet is not too long. Glue 36 grit sandpaper to the sanding block using 3M 77 spray adhesive. Sand until you hit glass, then stop. If you still have low spots, rough them up, fill them with dry micro and repeat the above until you have the smooth contour you like. Leave it in 36 grit scratches. Do not go to a finer grit sandpaper.
Now, mix up a little pure epoxy and, using a 6" wide soft rubber squeegee, spread this pure epoxy (no micro) all over the surface. The idea is to fill la of the 36 grit scratches with pure epoxy. Carefully squeegee as much of this first coat of epoxy aff as you possibly can. Use a lot of force on the squeegee and wipe the edge of the squeegee often with a paper towel. Allow to cure for two hours or so. The first coat should be gelling but not fully cured when the second coat of epoxy is applied in exactly the same way. Continue with this ritual until you have applied five separate coats. At two hours per coat, obviously you will need at least 10 hours at one stretch. Of course, this will depend on the ambient temperature and on what epoxy you are using. Here in Mojave in the summer, using Safety-Poxy or PTM&W epoxy, two hours between coats is sufficient. Allow these five coats to cure for a full 48 hours.
At this point, you have filled all of the 36 grit scratches and you have a very thin film of cured epoxy over the entire surface. All that remains now is to final sand. You should not have any runs or thick lines of epoxy. Wet sand with 220 grit followed by 320 wet. You are now ready to paint! That's it, no Featherfill, no Morton's eliminator, nothing but epoxy all the way. This way, there are no pinholes, no voids, no place for a delamination to start, no place to trap moisture. All you need now is a quality paint. I would suggest at least a high quality urethane or epoxy paint. Keep in mind that your composite airplane is very flexible and will flex in turbulence and while taxiing over bumps. If you use a brittle paint such as enamel or lacquer, it will crack at all highly stressed areas.
For the toughest, most long lasting finish, you should use the same epoxy for the contouring method described here as you used to manufacture your airframe. However, this may be time consuming because sanding Safety-Poxy micro can be very hard work. The only way to speed this up, for those of us who are impatient, would be to use the fast West System (Gougeon Bros.) for the contour job. It will go much quicker, perhaps only one hour between coats, and it will sand much more easily - it will not be quite as tough, but it will certainly be adequate.
I would appreciate any feedback from anyone trying this system. - ED.