Nope, first off Poplar is an interior wood and really for outdoor use. It will rot rather quickly and not as durable as you would want for columns.

Millwork post wraps over steel or other structural column would have been the better choice for a fine finish, as any massive solid timber undergoes significant seasonal shrinkage, which manifests as unsightly checking at the surface.
IMO the best filler to use would be Flextech by ART. It's a flexible gel epoxy that moves with the wood but can be sanded pretty smooth.
Casey

Thanks for the information. Would the Flex-Tec be suitable to fill in very long cracks? Several of the columns as well as the rails on the porch have long deep cracks in them. We are looking for something to fill in these cracks. We don't want to have to replace all the posts and rails. We're not looking for a perfect smooth finish; just something without all the cracks.