michael wrote:

. . .I am looking for a wetting agent. Some have suggested using washing
up liquid,but I do not know in what concentration and someone else has
advised me not to use it anyway.

I use non-biological washing powder rather than washing up liquid. It's
designed to help the water penetrate fine materials and it doesn't
lather as much as the liquid so one can use more, though the amounts are
still small. I use about 1/4 teaspoon per litre, but one can
experiment. Try adding measured amounts of powder to water and testing
how it feels on your fingers. You should be able to feel when the water
becomes silky without becoming soapy, then you can scale that up to the
amount you need to spray.
HTH

brian mitchell