The ornamental design for a paintstrainer, as shown and described.
FIG. 1 is a top, front and left side perspective view of a paint strainer embodying my new design, a paint can shown in broken lines and not forming part of the design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a right side elevational view thereof, the left side elevational view being a mirror image thereof;