Larry emailed me for tips on how to skin bare metal aircraft but a lot of this stuff is hard to explain using only a text description. So I decided to do a full tutorial with screenshots and post it in here as it might be of help to other skinners. I don't profess to be the best at metal effects, Fester and others do this stuff better than I do.
I start with a base aluminium layer of RGB 192/192/192. Over this I create three layers for green, blue and white reflections. The idea is that the shiny metal reflects the blue sky from above and the green ground from below and the white areas are highlighted reflections of the sun. The LH screenshot below shows these layers at 100% opacity as they look when they are being created. The RH screenshot shows the effects once the layer opacities are reduced down to 5-10%. This sort of thing looks a bit bogus in the game if it is overdone, as the reflections don't move when the plane changes its attitude....so keep it subtle.
Next I create three panel variation layers as shown below. The metal used to make real aircraft was sourced from different batches and so the aluminium panels often varied in colour slightly to their neighbours, you can often see this effect in photos. The LH screenshot shows individual panels painted in dark blue grey, light blue grey and light purple shades. I use a seperate layer for each colour as its a lot easier to change the colours later for future skins. The RH screenshot has the layers' opacity reduced until the effect is only just noticable. Note don't apply this effect to P-51 wings as the Mustang's wings were painted silver.
My panel lines, fasteners and hatches have a drop highlight layer to give a 3D effect, basically just a white copy of the panel lines offset one pixel horizontally and vertically. On bare metal skins I make a copy of this layer and place it below the camo/markings layers and above the base aluminium layer. So on the painted areas you see just one faint (15% opacity) drop highlight layer and on the metal areas you see both (15+70% opacities) giving a much stronger highlight on the shinier metal. This effect can be seen on the screenshots below. Note when doing 3D effects make a test layer of direction arrows showing a consistent light direction for each part of the skin, on my skins light comes from above, from the front and from the left.
I also like to create some aluminium sheet deformation effects around the rivets. To do this I make a copy of the rivet layer and paint it some bright colour, say red, to make it easier to edit. Then I offset the rivets in this layer one pixel horizontally and vertically in the same direction as the panel line drop highlight layer, i.e. away from the light source. Once all the sections of rivets have been moved, I turn the layer white and gaussian blur it by 1 pixel. I then do the process again for another layer but with the rivets offset in the opposite direction and turned black. These two layers become the highlights and shadows of the ripples in the skin around the rivets. The screenshots below show the effects in full on the left and reduced in opacity on the right.
The screenshots below shows all the effects together, along with weathering and 3D effects. Although I paint the main rivet layer black on bare metal skins, I've added some light grey rivets above just the painted areas of the skin, partially erased to give a worn apperance.
The individual effects shown above are not really noticable once the whole lot is mixed up, however combined they do stop it looking like an aircraft that's been just been painted light grey.
AH doesn't support specularity maps that would let a skinner alter the specularity (shinyness) of different areas of the skin. Most AH skins that have bare metal schemes do have material file support now, offhand I can only think of the B-26, P-40 and C-47 that don't. The material file lets you alter the overall specularity of the skin. However it is tough to get a good looking compromise between making both the shiny metal and dull painted areas of the skin look right. Below is the material.txt file I use on bare metal skins.