Three questions:
First off, what kind of MIG welder do you have? Your mileage will vary depending on the quality of the welder you are using.
Second, what kind & size of wire are you using? Flux cored wire will be a bit bigger, require a bit more heat, and cause significant splatter compared to solid wire that is shielded by an inert gas. Also, the wire needs to be sized for the weld. Too big a wire and you won't get good penetration. Too small a wire, you'll have the same problem.
Third, what kind of gas are you using (this is directly related to the solid wire)? Some wire, you can get away with CO2 or 90%Ar/10%CO2. Some wires will need 100% Ar.
If you are competent at TIG welding, I'd highly suggest that you rent a TIG welder for the project. MIG, IMO, leaves a bit to be desired in the area of finite control. Sure, MIG can produce the same results as TIG, but the skill level needed for that level of control is far higher than what a average person with a TIG welder is capable of.