Well, I was looking at the entries for Oscar and Louis on the 1900 Census - available for free here:

http://search.labs.familysearch.org/recordsearch/start.html#...

and, initially, after discovering more about the "outsider" artist Clarence Schmidt (born 11 Sep 1897), I assumed that the 1900 enumerator had transposed the birth years for Louis's sons, Clarence (born Sep 1894) and Percy (born Jul 1897). But, in double checking the census for other Clarence Schmidts in Queens, I found Oscar's son Clarence was born in September 1897, and so he is a much more likely candidate for the Woodstock artist.

Even though Schmidt is a common name, Queens wasn't SO densely populated in 1900 - and, on top of that, how many would be masons and plasterers? But it's all still a lot of guessing...

Marriage and death certificates might clear things up.

The Brooklyn Daily Standard Union (01 Jun 1929) reported that a marriage license was issued in Queens for:

Susan SCHICKLING, age 36, of 2521 31st ave., Long Island City
and
Clarence Schmidt, age 37, of 419 Webster ave., Long Island City

I'm assuming THIS Clarence is Oscar's son as that branch of the family lived on Webster (now 37th) Avenue in 1900 in a house Oscar owned. The ages seem a little off, however. And I can't find Clarence the artist's wife's name, so we'll have to wait and see if the pieces fall into place or not.