My dad's Bismark project continues with the odd instruction to sand 2mm from the side of each aircraft hanger. These were assembled some time ago, so I assume this modification is an error in the original parts spotted late in the day when producing the series.
Losing 2mm from a pieces of MDF is some serious sanding and almost impossible to achieve accurately without mechanical help. Far easier is to replace the part with a new one the correct thickness. The Parker stash of thin wood is well stocked with suitable material.
The only problem is that the parts are fixed together with waterproof (blue bottle) Resin W.
No matter, leaving small pads of damp kitchen towel against the joins seems to have weakened them enough that the sides to be replaced could be gingerly peeled away. I'm not sure that the glue has been broken in all cases, but the join softened enough that the very top surface of the MDF was peeled off.