Particles of a fine powder of a thermoplastic synthetic resin (host powder) having a suitable broad range between its initial glass transition temperature (initial T.sub.g) and its final T.sub.g, are partially coated with an anti-caking coating powder (coating powder) by blending at an elevated temperature to form "mottled" particles. The elevated temperature is defined in relation to the initial and final T.sub.g s as: (initial T.sub.g -20).degree. C. but below the final T.sub.g. Mottled particles, without being encapsulated, have the unique ability to maintain their individual particulate characteristics at a temperature at least 10.degree. C. above the initial T.sub.g of the uncoated host powder, and flow freely under applied pressure. Such pressure is applied by a rolling drum which spreads the mottled particles onto a target bed where they are selectively sintered With a controlled high energy beam, usually a laser beam. Sequentially sintering contiguous layers of the mottled particles results in a porous sintered body. Such a body may be used to make a prototype of an article, which may be difficult or too expensive to machine, then to form a shell mold for use in investment casting. Mottled particles, when stored above their initial T.sub.g, form a bed from 10% to 65% more dense than a bed of uncoated host particles, even when the coating powder coats less than 50% of the area of the particles of host powder.