Hi Geof,
I tried your scenario using QWP 5.7 for SP2010 and it worked for me using the following syntax:
a. Custom field of type Text called CountryID
CountryID='CAN'
b. Custom field of type Number called SecID
SecID=1
c. Custom field of type Text called Country ID2
Country ID2='US'
Don't replace the space neither add parenthesis to the field name. Use parenthesis only if you have multiple conditions in the complex filter.
What's your QWP and SP version?