800mcg is kind of a minimum daily requirement in my book. it must be selenomethionine to take this amount daily.
as for painting, you can use the grocery store tincture for that. the wood alcohol in it evaporates quickly. in fact, anything with elemental iodine can be used to paint, including my SSKI 2%. any lugols will do, though some have found the regular 5% lugols too strong for some applications.