With my comments to Mike, it got me thinking about just basic sanding skills. I'm old and when I was a kid building toy airplanes, pretty much anyone that built knew how to sand properly. Unfortunately, even that rudimentary skill is dying in our ARF world. I know this thread is about finish and painting, but if it ain't right on the bottom, it ain't going to be right on the top.

If I were to ask most anyone thinking of building a plane if they know how to sand, they would most likely all say "well of course I know how to sand". But do they really?

Here is a classic mistake in sanding we have all made (or you will). Let's glue on a leading edge to our wing and let it dry. We now come back with something like 220 or 320 grit paper and start to feather the leading edge into the balsa sheeting on the wing. We get all done and find we have one little spot about two or three inches that we can feel a little ridge, so what do we do?...we start sanding it more and more and more and of course it never gets any better in fact now it's getting worse! But I am doing everything right..I'm using a sanding block!

What we now have is a valley or low spot behind the leading edge. So what do we do in desperation? Yep we get out the light weight balsa filler and load in a bunch in that low spot. Of course we are not happy campers, so when it drys, we lay into the filler to sand it level with the leading edge and guess what boys we now have another dip on the back side of the filler where it meets the balsa skin...ahhhhh...Uncle!

So what's the deal. OK it all comes down to density of woods. Keep in mind our leading edge is going to be a totally different piece of wood than our skin material. In fact, I can pretty much guarantee it came out of a different tree. So here is what we did. We used......wait for it.....too light of a grit paper with too much pressure! But you are thinking, this guys nuts. If I used a heavy grit paper, it would make it worse! Nope. Other way around. You see what happened was you were using too light of a grit paper and applying way to much pressure to get the paper to do it's job and cut. So what happened was you were unknowingly applying excess pressure to the lighter density wood (the wing skin) so it just did what it was supposed to do and took off too much material.

So here is how we stop that...we use a heavy grit paper first with...wait for it...low pressure. In this case I would have leveled the leading edge with something like 80 grit. "80 grit paper" you say as you run around the room like Doc Brown did in Back to the Future...1.21 Gigawatts! That's right, very high grit paper on a sanding block with very very low pressure. What you do is almost skim the top surface just knocking down the higher (and higher density wood) leading edge. So now that we have it down nice and level what do we do to get out the 80 grit scratches? Well we don't go back to 320 because guess what, we are going to sand in the dip again...think about it. What we do is just go up gradually with the next paper grit like 120, then maybe 220 and then to maybe 320. When we go up in grit (finer) we only need just a very few passes to remove the prior grit scratches. Remember we have already leveled the surface so keep the pressure down on the block or you will be right back with the same problem.

One other cool thing about the Min Wax, silkspan system is you don't have to even mess with those ridges in the first place. I just leave them alone. You see when you apply the Min Wax and silkspan, pretty much all the wood becomes the same density...ding, ding, ding...is the bell going off? So now when you level the silkspan after it dries, all of those little high spots just disappear and you can do it with 220 paper. Cool!

Lesson over....you are excused.