I just purchased a spare tank with the very intention of coating inside and going with a planned paint job on my S3/Daytona.
The used tank I purchased looked really good, but had one small bubble/crack on it - what I found was that when I exposed it, there appeared to be a filler material and THAT is what had bubbled and cracked - I think the expansion causes the issue as it is less flexible than the Acerbis plastic itself.
I just completed the Caswell coating yesterday.
It was a bit cool in my garage and the sealer (part A) was REALLY thick - could barely pour it out of the can. After mixing it became slightly less viscous but could tell that flow within the tank was going to be less than ideal. I used the blood red version - partly for whimsy, but also as aid to see where it was coated vs not.
Not sure if was the viscosity issue initially, but became clear with the suggested 1/2 measure, it was obviously not going to coat the entire tank.
So I mixed up the remainder - this time I put the sealer can on my heater for about 5 mins first, and that definitely improved the viscosity.
After mixing I poured in the remainder and got it pretty much covered on whole interior. I would suggest that - at least with my tank - given all the nooks and crannies adding to the surface area, there is no way that 1/2 mixture would have worked to coat it fully, even if it had been less viscous.
I went through couple of iterations of having the pump opening open then masked - it is easier to see where it needs to flow to get the coverage where you want it when it's open. But of course you can't tilt towards the pump plate opening if is uncovered. I also used a brush accessing through the pump plate, to help coat the vent tubes. Rubber gloves essential here!
I actually used my heat-gun to blow hot air into the tank, which made the material less viscous and allowed it to flow.
I would definitely recommend that if possible, you do in a well-heated area, and have the material at >70F for 24 hrs prior to starting the job.
It doesn't look 'pretty' but functionally achieved what I wanted.
It was touch-hard within about a couple of hours - I brought it into house at that point and left overnight in room next to the furnace - which is always warmest room in the house.
Today it is rock-hard (24 hrs) although it won't see gas for a few weeks at least.