Thanks Spiros (@fiveten) and John (@johnb).
I had a bit of time this morning before my wife woke and before walking the dog so I spent it trying out sanding the bottom side of the Thunderbolt. I started off with Tamiya 3000 sanding sponge which I have always liked for it's consistent sanding finish. However, I quickly found out that sanding Aluminum foil wears out the grit on the sanding sponge quite rapidly, even when wet sanding. It still works, but progressively takes more effort. I also noticed that sanding in one direction on the foil would leave a slightly shiny effect while sanding at 90 degrees to that direction left a flat finish.
Now, one of the challenges I currently have, and where this rapid wearing out of the sponge comes into play, is that I am almost out of the 3000 grit sponge. That is a fairly pricey product and I am have been searching for alternatives. I recently purchased a bunch of 3000 grit sanding sponges from a different company. At first glance, you would think they should be the same as the Tamiya sponges, but they are a bit more coarse. Not a lot, but noticeable.
I tried these on a different area of the underside and interestingly enough, this new sponge had difficulty giving a consistently smooth finish on the foil, regardless of how many times I sanded the same area. So I went back over it with the Tamiya sponge and it came out looking more like the wings (which I did only with the Tamiya sponge).
SO I can extend my Tamiya sponge material by first starting with this new sponge material, but I am still somewhat leery of using this on more visible areas. I also tried 2000 grit wet or dry sand paper which did get a more consistent sanding , but it still needed the Tamiya 3000 grit sponge to look the same as the wings.
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1. Older picture, but it shows the shininess of the aluminum foil before any sanding. Compare this to the other photo where I have sanded the foil.
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1. This surface has been sanded (at least the wings and forward lower belly.. It's hard to get a good photo of this that looks like it does in person. So this photo is not completely representative of the smoother, flatter finish after sanding.
I also first tried sanding the foils in different directions on different panels, but this was difficult to do and get all the way to the panel lines without accidentally touching other panels. So I am instead going to get a consistent sanded finish on the entire model, then use the oil filter washes and clear coat finishes I learned from testing to give the panel variety.
One other thing I learned in testing, which I forgot to mention, is that anything less than very low tack on any masking materials is at risk of pulling the foil off the model. This happened a couple of times on the test mules. So I will have to be especially careful with any masking I do.