So from your reports (porosity, not filling the mold) you were likely running into 2 very common issues, one easily correctible. For porosity, especially if yo

So from your reports (porosity, not filling the mold) you were likely running into 2 very common issues, one easily correctible. For porosity, especially if you were using aluminum, that is very likely from using the wrong alloy. Trying to remelt aluminum cans or extrusions especially will produce very porous castings. Start with a known-castable alloy, the easy way is to start with scrap that was previously cast. Look at the original part for signs it was cast, that doesn't tell you the exact alloy but it does tell you that it can be cast. For the not filling the mold, that is shrinkage. The metal contracts when freezing, which can cause the part to be undersize or have large voids. This is fixed by proper mold design, which is a surprisingly large topic and a bit of an art as well and can't be covered in a single topic. One thing to look into is adding a riser, a larger chunk that is designed to stay molten after the part has frozen to concentrate shrinkage.