Roofseal type stuff

I'm after something like Thomson's Roofseal to seal some old corrugated cement roofing boards which appear to absorb water and drip through. I was looking through the Screwfix catalogue and saw "Stabilising Solution Concentrate", "Waterseal concentrate" and "Flexacryl Roof Repair Compound". The concentrates look like a nice idea, as its only a couple of sheets which are a problem, and I'd prefer not to use a lot of the proper black sticky stuff as its a large roof, and 2 coats would go through it quickly (and at 20 a tub, that's not terribly cheap) - plus, it would change the colour and be a big job.
So - would any of those concentrates be suitable for trying to stop water penetration of old cement based sheets?
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