Painting continuous areas at different times and sanding curved surfaces

[Chris Martin]

Hello SPI Friends,

My project is going well but going slowly because of the high temperatures. But getting there.

I have a question: As you can see in the picture attached, I need to paint a fuselage that is one continuous curved surface all around. The fuselage sits very low to the ground and it would be hard for me to paint the whole thing one shot. Painters that I know do the bottom first then the sides and top after the bottom cures. This is easier to do in aluminum airplanes because there are plenty of skin seams to separate the jobs. But in a composite airplane there are no seams.

Any techniques the group can share that would allow me to do the flip the fuselage upside down, do the bottom, wait till cure, then flip and finish top. The two ways that occur to me is to create a line by masking then sand and buff. The other to try to blend the seams. I am using white SS.

The second question is what would be the best way to block such a compound curved surface? What blocks you think would work best? Should the block be held aligned longitudinally or circumferentially when doing the cylindrical aft part of the fuselage? When I did the initial blocking to prime 2 years ago I used shorted durablocks but not sure if that is the best technique.

Thanks for the advice,
Chris

 

[dhutton01]

I would try to rig some kind of a rotisserie that would enable you to paint it all in one session. As far as blocking it since it is a curved surface I would use 1500 grit dry on a DA with a soft interface pad. A quick pass then 2500 and 8000.

 

[Chris Martin]

Thanks for the advice dhutton01. I do have a setup that allows the thing to flipped upside-down but the vertical fin at the tail is quite large and it is hard to flip while in my narrow spray booth. Anyway, agreed it would be the best way to do it and will think about it more. But still considering a blend between top and bottom. One thing to keep in mind is that the bottom is really not easily seen (this airplane sits very low to the ground as you can see in the picture). If the bottom is not perfect it is not a big deal.

Regarding the blocking, I think you are talking about sanding the blended part of it. However, I still have to sand the old primer with 180 and the fuselage needs more filling and blocking since it is not really finished well enough. The fiberglass fuselage was originally a top and bottom halves joined on the sides. The joint still needs more blocking to eliminate waviness. Doesn't look terrible but when I spray glossy SS I think it will not be up to my expectation.

Thanks,
Chris

 

[Chris_Hamilton]

For blocks forum member Dean makes some nice ones.

Black Diamond | Flexible Sanding Blocks and More

blackdiamondblocks.com

You could also try using acrylic sheet in various thicknesses. Use some duct tape for a handle. Search forum member theastronaut for some pics of what I described. I'll try to take some pics of what I use today as well.

When you are blocking imagine a straight line running through the plane horizontally from front to rear. Crosshatch off that imaginary line. 45 degrees one way, then 45 degrees the other. For your final sand you want to sand in straight lines one way only. Don't crosshatch.

Like Don said only way to do it without lines is to rig up something that will allow you full access. Not being familar with the plane and it's shape it's hard to say something specific. One thought that came to mind is if you could lift the tail up it looks like you would have access to most of it. Or Fashion up some "blocks" to lift the entire plane up some so that you could access the underside.

Then stands like these to do the top part:https://www.creativecoatingsolutions.com/van-stands.aspx.

You'll be moving around a lot, moving the stands back and forth, will be some work to do it. Or if you could fashion a rotisserie that would be nice but I don't know how practical that would be. Another thought would be if you could suspend the plane off the steel beam in your hangar. I don't know if the beam would be able to take the load though. I would lean toward blocking the plane up some and using stands to reach the higher areas.

 

[Chris Martin]

Thanks for the feedback Chris.

I have read about the Black Diamond sanding blocks in the forum. Look like the way to go. I do have a sanding block that is flexible and made from an acrylic sheet. I will give that a try since I have it. When I tried it two years ago wasn't particularly impressed. Pic attached.

Thanks for the sanding technique info. That's what I needed since I wasn't sure.

Regarding the painting, I'm going to consider a way to rotate the fuselage. But what you describe is the other technique that is commonly used (get the tail up as high as possible and get underneath) since raising the tail is easy if you have the space. The issue is that the spray booth is not too tall but I may be able to raise it enough to work on the bottom (you can see it on one of the pictures of the fuselage, behind the airplane). But I can always modify the booth to create a pocket to let the vertical fin go higher. Since there is quite a bit of primer I have to spray I will practice spraying the whole thing at once with primer and see how it goes. And I am using the deKupss system which I think will help with spraying the bottom. I just have some doubts about my skill level But as I said, the bottom is truly not something you can easily see, just trying to do the best I can within reason.

I do have something similar to those Van stands (the aluminum ones from HF).

Thanks once again for the help,
Chris

 

[aviator8]

I'd try a rotisserie. Build a U and tie each side to the Tail horizonal stabilizer mounts. Then have the center of the U be the rotation position. They are designed to take load. I cant see the front. One option woul be to mount the other side to the prop mount but you'd need to check the vertical load specs on that. If that was good you can then build the leg hieghts for ground clearance with the tail being higher to clear the vertical stabilizer in the inverted position

 

[Chris Martin]

Here is a pic of my rotating setup. Spraying primer two years ago which I did outside since I didn't care much about contamination. I am going to remove the engine to make things easier. I could modify the fixture to have a center of rotation in the middle. Not sure I can spin the thing inside the booth. Well, will think about all this. Thanks.