Marques

If it´s difficut to sand it´s most likely PLA. It´s not ideal for modelling, just because of that: it´s difficult to sand and easy to melt. Be careful, it may be that if you sand too hard or with a electric tool the surface may melt and warp. A possible plan B is to fill instead of sand (with a coarse primer, diluted putty or car industry putty) although then you may loose detail. But personally I think that this is the best approach in this case.