Finally spring is in the air. The temperature swings between 6 and 9 degrees Celsius.
The keel has been sanded with sand paper grit 80. Rust spots have been sanded away to bare metal and painted with two component epoxy primer International VC TAR2.
Next step is to putty where rust has been sanded as well as firing the keel. In order to bring up the temperature I have hanged a plastic tarpaulin around the keel and held it down with some wooden boards. This reduces chilling effect of the wind, dispersion of heat inside the space as well as greenhouse effect since the sun is still relatively low and light goes through the plastic.
A air heater has ben placed inside the space to reduce humidity and increase air temperature whilst an infrared heater set to warm up the huge iron keel. The effect of this last step is crucial as it increases induced warmth on the surface where the putty will be laid as well as making the keel work as a radiator and holding warmth up over night.
I have used West System putty with fast hardener that works down to 5 degrees Celsius. I found it very easy to apply with a small putty spade and a large one for fairing. The dough must be thick enough to to stay where it is spread. The putty was workable for about 20 minutes before going to gel phase and no longer applicable. To facilitate sanding afterwards and increase consistency I used microballoons mixed with the putty. It required quite a lot of it to provide the right thickness. West System sells separately pumps to provide the correct amount of base and hardener. Works wonderfully!