I managed to retrieve Jims missing post and have pasted it here. Thanks Jim- this information is far too good to leave out.
Jim has left a new comment on your post "On rendering a water tank":
Ulf that is a lot of hard work but at the end of it you will be happy because you have been able to recycle something and there is nothing better than drinking water stored in a concrete tank.
I have rendered 5 tanks and cut my teeth on another three.
The first tank I was knowingly involved with was when I was about 6 or 7 by which time Dad had already done 2 others only a couple or so years before hand (I just don't remember them but have photos to show I was around from about age 3). His first two were 2,000 gallons each and the next was 1,000 gallons. I was involved with 2 other 1,000 gallon tanks during my teens.
Move on to nearly 30 and my wife and I had just moved to a new job and "new" home. The water setup was terrible and the first holiday I had was christmas day and we prepared to render a 1,000 gallon tank. It was very hot and exhausting but we got the job done in the day, even carrying sand and cement up a bank and mixing it by hand on a sheet of flattened iron. Over the next 15 years I rendered another four 1,000 gallon tanks.
We never used chicken wire to reinforce the inside and only made the render 25mm deep on top of the rungs.
I painted bondcrete (or cemstik which is only pva glue) inside the tank on the iron surface a day or two beforehand then mixed about 5 drops into the render mix. Also in the render mix which was in the ratio of 3:1 sand and cement were 2 drops of liquid detergent. I have never used lime but a lot of mixes do recommend it.
As you say start at the bottom and move up working on 3 or 4 rungs at a time. A trick I worked out was I would fill in the valley all way around then follow up with the top coat finishing it off before moving up to the next 3-4 rungs.
Once it starts going off mist a little water on the lower surface so it doesn't dry too much. After the job is completed I always misted water over the walls every couple of hours during sunup to prevent over drying too quickly because it is only a thin coat of render and the galvanised tank too picks up a lot of heat.
On one tank we had some cracks develop (probably slight slumping) and I painted over the fine cracks with a cement/water paste.
After about 5 days I would fill the tank.
As far as I know the tanks I did some 15 to 30 years ago are still going and I am sure the ones done 50+ years ago are still serviceable too.