I have tried a few of your solutions. Vari-ND is based on stacked polarisers, but I have not used one. I have stacked various polarisers that I had lying about, some £10 from ebay and some old linear versions. I have tried all the combinations but have not had good results. Magenta casts and cross shaped effects, where the cross is dark magenta and the corners outside the cross are lighter. Welders glass is great and cheap at £4, but it is VERY dark (the one I have is about 20 stops) and green. You can correct most of the colour cast, but you have to ensure that it is clean and scratch free or you will get flare effects around defects. Also, light leaking behind any of these is an absolute no-no (so makes cokin filters a bit of a trial to control). Light leaking back through the viewfinder is a real issue as well (with my Kx and K100,) so block the viewfinder. Cokin & IR is fine, but it will be IR, so not really realistic. I had a good example of the Citiwide ND400 which I gave away. It had a magenta tint as well, but was interesting to use and probably 15 stops rather than the rated 9. Sadly the new versions of that filter were faulty (creating the same magenta cross as bad crossed polarisers, so you can guess how they made it. It is no longer on sale, but if they can fix the problem I would buy another, they were only £13). So all those will cause you problems, and you are left with the expensive solutions. I have a B+W ND110. The 58mm was
only £30 I think, the 77mm was closer to £80, but they do actually work well for landscape. There is a slight pinkinsh cast, but it is easily correctbale. I feel that I wasted a few quid on cheap attempts, but do not regret getting either of the B+W ND110s I have.
This is one with the welding filter (this was just to try it out, I would have preferred some moving clouds or something to blur)
There are some with the B+W here
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