I spray all my moulds inside and out, with Lyman moly bullet spray coating. It works to prevent ANY soldering.

Candle flame does the same thing and might be a good alternative.
I like the moly spray as it's a very thin coating & does not change the diameter of the cast ball (lead).

BTW - if used in a ML, the ball diameter should be .020" to .030" or so smaller than the "bore" diameter to allow a thick enough patch to be used to fill the grooves. This is due to the ball not 'conforming' to the lands. It will be non compressible. The patch must also hold enough lube to soften the fouling of the first shot and the loading of a second if the second shot is required.