Sanding

Over the years, we have learned better sanding techniques than were called out in the original plans. I'd thought I'd cover some of the main sanding tools that I now use. (As always, click on the photo the the full size image).

Again, the main idea is to lay your filler material on THICK, then sand down the high spots until you just reach the fiberglass. There are two excellent on-line descriptions that describe best practices.

Chapter 25: The Contouring Process

and

Finishing a Composite Airplane

1. Sand Paper The first strong recommendation is to buy only automotive body shop quality paper. I think that the 3M paper is the best and longest lasting and is my preference, by far. I get my paper at a local automotive paint shop supply store.

Almost all of the work will be done with 36 grit paper and I keep a good supply on hand. I really like the stuff that comes with the adhesive backing. It's 2" wide by 16" long.

2. Small Sanding Blocks Try and avoid doing any sanding, at all, with just your fingers - always use a sanding block. If you use solely your hands, the paper will follow the contour of the thing you are trying to sand down and you will not get a smooth surface.

I make extensive use of the rubber sanding blocks - there are both 6" and 8" available. I stick the paper to both the curved and straight sides.

For fillets and the like, I simple stick some 36G on a piece of cardboard tube.

I have also used this single stick board for many years.

3. Long block I used some shelf board I had laying around and cut it to 32" and mounted a section of aluminum angle for a better grip. This length allows me to use two 16" sticky back strips. With fresh paper, this is a great tool and really cuts into the micro.

I strictly use 36 grit on the whole plane until I get the contours that I want. When I have that desired shape I switch over to 80 grit. All the 80 grit does is to knock off the highest cuts left by the 36G.

All sanding should be done at 45 degree angles.

Don't worry about deep scratches, pin holes, deep holes or black holes. The next step will take care of it all,

Next, comes the epoxy wipe!

Time to Start Filling and Sanding

To keep this post short and simple, let's just say I used the advice from George Sychrovsky and Wayne Hicks. Be sure you read those two links if you are going to finish your plane. The Prime Directive is, "ONLY FILL ONCE"!!! If you and filling, sanding, filling and sanding, you are doing it wrong. I used their method this time around and am very glad I did. However, I did not go the black primer route.

I used West System, which is non-structural for aircraft, and glass microballoons. First, I filled low areas using the fast hardener. Then I started mixing and mixing and spreading and spreading. I really lathered it on. I bet some places were at least 3/8" think. But again, the idea is to lay it on only once and sand down to the low spots. It took me about 1-1/2 hours per wing panel, doing one at a time.










After about 5 hours of cure time I can go at it with a Stanley Surform file to knock down the high spots.



I used two-foot sanding boards with 36 grit paper. I use the 3M stuff that already has the sticky back. Per George's recommendation, I made the boards from laminated bookshelf boards I got from Lowes, then attached a piece of angle aluminium on the back. Before got really into it, I took a pencil and really marked up the surface. This helped me gauge how much I had sanded and identified the low spots that I would sand down to.

Sand, sand, sand for a couple of hours. Note the pile of dust. Really though, it's not that hard. I just say to myself, "I am going to work on the plane from 9:00 to noon, and when it's time for lunch I will have a sanded wing panel"! I bet I sanded off over 80% of the micro that I applied.

When it looks good with the 36 grit paper, I switched to 80. Just a few minutes with the 80 and that's it! At this point there are all sorts of awful holes, crevices, and scratches. Previous methods would have had us filling and refilling. Yeaucch.

Next, I do the epoxy wipe. Mix up a full cup of West and squeegee it on. I just apply enough to make it wet, then try and squeegee it all off. I am not building coats, I am filling the imperfections. I repeat this every hour until I have applied 4-6 coats.


Next say put some 120 grit paper on the sanding boards, make a few passes and I'm ready for primer! I love it!