I'm pretty sure I've already posted pictures of the "finished part" version of this, but I wanted to show how I fabricated the part. I used my new heat exchanger part as a mold. I wrapped it in clear packing tape and then used clay to form the fillet around the pipe and generally smooth out the transitions. After I had the clay where I wanted it, I used the PVA mold release (the green hue) as a parting agent. I then laid several layers of 6oz. fiberglass cloth over the whole shebang. Once it cured, I popped it off, trimmed it and eventually glued it back together.

After a bunch of sanding and filling with SuperFil, the lower cowling was finally ready for its first coat of primer. The blue/green tint you see is a result of the SuperFil filling the weave in the fiberglass.

Nobody really intended the inside of the cowling to be filled and painted to the same level as the exterior so the original fiberglass layup didn't have any peel ply or gelcoat. As a result, it took a fair bit of sanding and SuperFil to get it smooth.

I sanded the exterior of the cowling a fair bit before applying the primer. You can see that the gelcoat is no longer shiny in this picture. I just prime things outside my hangar door when the weather is relatively nice and the winds are calm. I figure everything will be sanded anyway so I'm not that concerned with dust/bugs/other impurities.

I don't remember mentioning it recently--I use an Evercoat brand product called Uro-Fil for my composites priming. It sands well and seems to fill pretty well, too.

Here's the lower cowling wearing its first coat of primer. If you click on the picture and look at the enlarged version, you can see a few places that will require more filler and an additional primer coat, but overall I'm pretty darned happy.

It's always rewarding to spray the primer and see your seam work disappear forever (hopefully!).

I primed the top half of the cowling, too. Again there are a few imperfections that will need to be addressed, but nothing significant.

I failed to take any pictures of the inside of the cowling halves, but they're also primed. I'm pretty happy with the way they came out, too. I'll need to do a little more filling, sand the whole thing with 400 wet/dry and then prime one more time before the final paint. I'm going to paint the inside of the cowling white. I like white because it looks clean and you can see any leaks right away. Painting the inside of cowlings is an absolute must in my book. I hate the looks of unpainted interior cowlings after a few years' worth of drips and botched oil changes!

The next big step in working with the cowling was getting the cowl cheeks to fit. Unfortunately, I should have fitted the cheeks at the same time I fitted the main part of the cowling. If I had done this, I could have squeezed the edges of the main cowling a little tighter and trimmed along the joint where the top and bottom halves of the cowling come together. This would have caused the aft part of the main cowling to fit snugly against the flange on the cheeks.

Short story is that I didn't think to do this until after the two halves of the main cowling had been fitted along with metal flanges installed and camlock fasteners riveted in place. To remedy the error I decided to start by cutting off the molded-in flanges on the cowl cheeks and replacing them with aluminum flanges.