This will be tough because each product can actually have up to 3 custom fields. The custom1, custom2 and custom3 fields only hold the record # pointing back to the customfields table. The content1, content2 and content3 fields in the products table actually hold the values that you probably want to sort on.

If you only have one custom field per product then you could try sorting on:
"products.custom1, products.content1". But this will sort by the custom field record # then the custom field values. If you only have one custom altogether then you can just do the sort on "products.content1".

If you actually want to sort by the real custom field names you would need to modify the whole SQL statement to join the customfields table. That could cause your searches to slow down. I would recommend an outer join so you don't lose records that have no custom field values.