Water Tank Repaint - Conditions

[rupertbear]

I intend to repaint the inside of my integral steel water tank in the near future and have been wondering about the best time to carry out the job. I want to get it right because the previous job (not by me) appeared sound but I have discovered that it had not adhered properly and much rusting was going on underneath.

I know that conditions leading to condensation on the tank surface should be avoided. Beyond that, I start to think about air temperature, (canal) water temperature and temperature of the steelwork generally then very quickly become confused.

I wonder whether it might be that not only the relative temperatures might be important but possibly also whether they are rising or falling?

The tank will empty so most of it will be above the water line. Since steel is a pretty good conductor of heat, does this mean that the water temperature is of little significance compared to that of the shell overall?

In practical terms, given a warm dry day, at what point should I paint? Earlier, as the temperature is rising? Peak shell temperature? Some other time?

[David Mack]

Sure I've read somewhere that you should paint when the air temperature is above 5 degrees and rising. If you paint later in the day, then you will get condensation on the paint which will cause a bloom on gloss paint. Not sure it would matter with tank paint though.

[Chewbacka]

If you are using epoxy, best to paint in the shade or on an overcast day as the sun will make the sides and top of the tank warm - the side facing the sun may even be hot.  The epoxy will cure too fast, including on the brush/roller so you will have only moments before it goes hard.  The base of the tank being at canal temperature will cure a lot slower than the sides in warm weather.  I used a painters/chemical dust mask as it has a carbon filter to save you from breathing solvents into your lungs.

[rupertbear]

Thanks for the replies.

Update:

I remembered yesterday that I had previously bought an infra-red thermometer from an Aldi special offer, so I dug it out. Weather on my mooring was hot and sunny all day and I took some readings around 5pm. The circumstances were extreme but I was still amazed to find a difference of 17 degrees between the small section of the tank below the waterline and that above - 19/36! I had expected the good thermal conductivity of steel to have evened things out much more than this.

I would not, of course, attempt to paint in conditions anything like these but it does suggest that any time that the air temperature is much above the water temperature there may potentially be condensation. But it will also depend on the humidity level?

[blackrose]

If you're just painting bitumen then anything above about 10C should be fine. It really depends on how well you're going to do the job. If you're prepping properly and going back to steel then 90% of your work will be the the prep and just 10% the painting. In that case it seems a shame to use bitumen which will only last a few years and I'd recommend a water potable epoxy paint such as Armourguard PW (which is primed with a couple of coats of non-potable Armourguard ST). When using this system then you need to think about your prep, painting conditions, curing time between coats, etc. However, if you're just using Vactan (rust converter) and bitumen I really wouldn't worry too much about it.