AllSeeingSkink wrote:That's actually similar to a technique I use to weather aircraft... I didn't think to mention it in this thread because my mind was stuck on Nuln Oil.
I'm not sure how well it'd work on Space Marine tanks though, it works well on aircraft because aircraft have large curving surfaces which you can work with a tissue to clean up the wash, as there's a lot of rubbing involved
I use oil paints rather some fancy paint specifically designed for the task

You thin them down with white spirit (I don't thin too much, the more you thin it the more it'll tend to cling to the surface and won't rub off as the white spirit tends to dig in to the underlying surface), apply it over a gloss surface, wait a few minutes for it to start to dry (I usually just do 1 wing at a time on an aircraft). Then grab a tissue and start rubbing it off. Keep rubbing until you're happy with the results.
However it's more of a method for doing panel lines in aircraft than anything,
I used that technique on this wing, it's how I darkened the rivets, added the hint of gun smoke staining and also how I added the brown dirty look in to the wheel well and around the control surfaces (the panels toward the trailing edge of the wing). I adapted the technique from PLASMO's youtube channel, he uses it a lot on his aircraft. The main difference between my technique and his is that I don't thin my oils as much, I put them on as thick as I can and still have them fall in to the panel lines and rivets (if you don't thin at all they tend to sit on top of the panel lines instead of going in to them). That was I can create the blending effects you see in the wheel well and control surfaces. But otherwise the technique is the same.
This video at 8:20 shows it:
https://youtu.be/prPeSlciaLY?t=8m24s
Or this video from around 13:00:
https://youtu.be/RSzNMbrnR44?t=13m1s
I'm not sure it's going to give you the effect you want on a Space Marine tank though.
I still reckon you can get a Nuln Oil was to work, if I can find a vehicle to test it on when I get back from work I'll have a go. One thing that might work that I experimented with last night (not in the context of tanks, but again trying to get smooth finishes on aircraft) is mixing Army Painter inks (basically the same as
GW's old washes) with Humbrol Acrylic Thinners. HAT have brilliant self levelling properties, it massively slows the drying, but the wash goes on much smoother. I haven't tested it on a large surface like a tank though.