Seams in a 6' dish won't have ANY noticeable affect on how well it works. The total area attributable to these seams can't be more than a half percent of the total area of the dish, so losing less than 1% of the possible signal is pretty much meaningless. You lose more than that from the feed support arms and feed blocking some signal, and it's all made up by the increased size of the dish.
And even the biggest mesh holes I have ever seen on a mesh dish won't have more than maybe 5-10% effect, and if you have a dish that is bigger, you more than make up for that via the increased size. I think this whole rule of thumb thing with the pencil or pencil lead thing has been blown way out of proportion, and actually, the hole size isn't even the main parameter used to determine loss due to mesh. Actually it's the mesh spacing divided by the diameter of the wire or metal, and the spacing divided by the wavelength. Of course the hole size is related since the spacing is hole size plus wire size, however big holes can be offset by bigger wires. My old 10' Orbitron has holes that are about 3 mm and the metal is about 0.4 mm. That 3 mm is bigger than any pencil lead I have in the house, but when new, that Orbitron performed significantly better than my 3' dish. It's main problem on Ku was that the low F/D made the feedhorn only see the middle half of the dish. If you use commonly used nomographs with the above data, it predicts about a 0.28 dB loss in gain due to mesh size, with only about 7% loss of signal. That 7% loss of signal is more than made up by the increased size of the dish.
Bottom line is that by far the biggest parameter relative to whether a dish will work on Ku is the SHAPE of the dish, and if the shape is bad, it's not going to do any good to cover it with finer mesh, screen, reflective film, because the shape will still be bad. If the shape is already good, you're only going to make it worse by messing with it, and the most you are likely to gain is 5-10%.
Now, if you do the pencil test, and the whole pencil goes through, not just the lead, then the loss gets more significant, but it's still not like you lose all Ku, you'r just going to lose some percentage of the signal, and if the shape is good, that can be made up by the increased size of the dish. But finer mesh or increased size is NOT going to make up for bad shape.