Change Data Type/ Join Mismatch

Looks like mismatched join types has caused others plenty of headaches in the past!!
I am trying to create a query that connects 2 separate areas of our factory.
The first table (Table 1) has a field called prod_code which is a text field and this code (5 digit number) describes a manufactured product.
The next table (Table 2) also has a field called prod_code which is also a text field, is a different 5 digit number, and describes the same product but after packaging.
There is a 3rd table which I want to use to link both of these tables, but in table 3:
table 1. prod_code = table 3.item_code, and is a long integer. and table 2.prod_code = table3.item_code_prnt, also a long integer.
I need to be able to use table 3 to correlate data from tables 1 and 2.
I suspect I need to use Clng or similar but am unsure how to apply it.
Have been trying to construct an expression but continually get error (bracketing error, join mismatch etc, etc)
Any clues?