Re-using specified folder later in an AppleScript

I have an AppleScript which involves the user selecting a folder for images to be exported to (from Aperture). Later I need to run a shell script on the images in that folder. I had help at https://discussions.apple.com/thread/3012302 to get the shell script running. Currently Aperture exports the images but the script runs with an error

error "No file specified" number 1

suggesting I haven't said which folder to use.

So I believe I need to "tell FInder" the location of the folder chosen by System Events. If so how do I go about passing the name and location of my 'exportFolder' to application Finder? Or do you think something else is causing the error?

Script below, many thanks!

-- Creating a new filename by making Version Name from the IPTC Headline and Filename (with hypens where spaces exist). The Aperture Version Name is used to create the file's name on export. Once exported the Aperture Version Name is reset to its original.

tell application "System Events"

set exportFolder to (choose folder with prompt "Choose an export folder")

end tell

tell application "Aperture"

set theSel to (get selection)

if theSel is {} then

error "Please select an image or two!."

else

repeat with theImg in theSel

-- Creating a new Aperture Version Name which will become the exported file's filename.

tell theImg

set headline to (get value of IPTC tag "Headline" of theImg)

set AppleScript's text item delimiters to " "

set theTextItems to text items of headline

set AppleScript's text item delimiters to "-"

set headline to theTextItems as string

set AppleScript's text item delimiters to {""}

set objectName to (get value of IPTC tag "ObjectName" of theImg)

set newVersion to (headline & "-" & objectName) as string

set name of theImg to newVersion

end tell

end repeat

-- Exporting the files as JPEGs to chosen folder/Project Name using the Version Name as a filename

export theSel using export setting "JPEG - Original Size" to exportFolder

-- Resetting the Aperture Version Name back to filename using IPTC Title (which should be the file's filename without suffex).

repeat with theImg in theSel

tell theImg

set title to (get value of IPTC tag "ObjectName" of theImg)

set name of theImg to title

end tell

end repeat

end if

end tell

--Using ExifTool to set Photoshop Copyright Status and fiddle some EXIF camera data

tell application "Finder" to set theFiles to files of exportFolder as alias list

repeat with eachFile in theFiles

do shell script "/usr/bin/exiftool -overwrite_original -Photoshop:CopyrightFlag='True' -Photoshop:URL='http://davidgordon.co.uk/'" & quoted form of POSIX path of theFiles

end repeat

display dialog "Done that!" with iconnotebuttons "OK" default button 1 giving up after 10