Another thing that helped a lot with molding and casting is Petrobond molding sand. It is an oil base sand and is very fine. It allows me to mold any time without tempering the sand and the main downside is a little bit of the sand is lost with each cast. It can be saved and remixed with the oil, so I just keep it separate from the unburned sand. It will yield very fine details in the cast parts as well. It probably costs a bit more than green sand mix but the convenience made it worth it to me. If the voids do not affect the main working part of the casting you may be able to utilize it even though it cosmetically looks bad. I did not see the core area on the top of the carriage, did you opt to make it without that?