In the antiquity, the traditional compound the organ builder used here
in Italy was a mixture of hide glue and red earth.  More recently, we
use gum arabic mixed with kaolin; here is a recipe:

1. Take 40 grams of pure kaolin, and dilute it well in 135 grams of
hot water, then let it settle for almost 12 hours.

2. Contemporarily, mix 40 grams of gum arabic with 200 grams of water,
and let it settle as well separately.

3. After the due time has expired, mix the two solutions well, then
filter first with a strainer, then by stocking fabric.

The solution is ready.  If you want to store it for a long time, add
a preservant.  A good preservant is sold here as "Nipagina"; I don't
know if it is named differently elsewhere.

The metal sheet should be well degreased before applying the soldering
mask.  Very old lead pipes are usually covered with a brown-green
oxidation layer that is sufficient as a protection in restoration;
tin pipes need a protection.

Also, if quick, small soldering is needed in restoration, the borders
could be protected by a sheet of adhesive paper.

As for the solder, I usually use 60/40 [60% tin, 40% lead] that I
prepare myself.  It allows one to use the lowest possible temperature.
Anyway, in the celebrated treatise by Dombedos, different combinations
are suggested for soldering different parts of the pipes; I don't
remember the details now.

Don't forget to use stearic acid as a fluxant.  Hope this helps.

Best regards,
Leonardo Perretti
Italy