How to recode a FOR Loop so that the variables in the FOR Loop are assigned to a StringBuilder object that is created outside of the FOR Loop?

Well if you want to write the contents of the StringBuilder to the StreamWriter you need to change this:

sw.WriteLine(stringb);

To this:

sw.WriteLine(stringb.ToString());


To prepare the StringBuilder for the next iteration through the loop you need to clear the context of the StringBuilder by doing this:

stringb.Length = 0;

That should do it.

I don't seem to be generating any of the variable values in my output, all the values are BLANK:

For ex:       stringb.Append(checknum);
                  stringb.Append(routingtransit);
                  stringb.Append(bankacct);
             

COMMENT: specify code page of the index date
CODEPAGE:819
COMMENT: CHECK NUMBER #0
GROUP_FIELD_NAME:CheckNumber
GROUP_FIELD_VALUE:                            <-------    checknum
GROUP_FIELD_NAME:RoutingTransit
GROUP_FIELD_VALUE:                            <-------    routingtransit  
GROUP_FIELD_NAME:BankName
GROUP_FIELD_VALUE:BANK
GROUP_FIELD_NAME:BankAccountNo
GROUP_FIELD_VALUE:                            <-------    bankacct
GROUP_FIELD_NAME:CheckAmount
GROUP_FIELD_VALUE:
GROUP_FIELD_NAME:CpcsNo
GROUP_FIELD_VALUE:00000000
GROUP_FIELD_NAME:CheckPaidDate
GROUP_FIELD_VALUE:
GROUP_FIELD_NAME:OfficeNo
GROUP_FIELD_VALUE:

Then the way I understand your observation is that I cannot build the entire StringBuilder structure beforehand due to the variable values.

Only the constant header values can be contained in the StringBuilder before entering the FOR LOOP.

As you can see, virtually every other record contains a variable value, so how could I place all the FIXED column header text into the StringBuilder before entering the FOR LOOP while leaving alternate records to be appended inside the FOR LOOP.  

For ex: It seems to me that I can only place the following 2 lines in the StringBuilder and then the rest of the records would be appended inside the FOR LOOP.


2 lines in the StringBuilder:
StringBuilder stringb = new StringBuilder();  
stringb.Append("COMMENT: CHECK NUMBER #");
----------------------------------------------------------------------------

then in the FOR LOOP
----------------------------
CheckOrderNum = CheckOrderNum + 1;
stringb.Append(CheckOrderNum);
stringb.Append(Environment.NewLine);

stringb.Append("GROUP_FIELD_NAME:CheckNumber");
stringb.Append(Environment.NewLine);
stringb.Append("GROUP_FIELD_VALUE:");
checknum = s.Substring(246, 6);

stringb.Append(checknum);

What is the source of the data is it a single database row? If it is and you are iterating through the columns and know the column names but for some reason can't get the column names at runtime eg you are using ordinal position, you could use string builder to build a string pattern e.g
X.appendline("name:{0}")

Then once you have all the lines just do some thing like this
String.format(X.tostring(), row[0], row[1])

This does seem a bit clunky though your better being more oo create an object like a view model approach with matching properties and override to string with something injected so you can unit test it easily and the data isn't couple to your view or string representation, think about if this is plain text for and email what happens I you are asked to make it look good and send it as HTML?

Another route is your dealing with key values here? Why not use a collection iterate like so
Full collection from data source column name as key, field as value
Foreach key in collection. Keys
X.appendline(key + collection[key])

I am reading a binary check image file and I know the field values that I need to retrieve based on their displacement after a specific record type field value. For each check, there is another record of this specific record type.

I would create an object that contains those fields with the exact names, populate the object and you could be clever using reflection and output the property name and value in an overriden tostring