Steve is right, but I'm sure that what you should have done is not the most important thing to you right now. You can go over the blotchy stain with a darker gel stain color. You should be able to do this and achieve a much more even color. Gel stains do not soak into the wood the way dye stains are supposed to. And sanding hardwood plywood can be trecherous. If you sand too deeply, the paneling will never look right.
At the same time, many gel stains do not offer the wood grain pattern that liquid stains do. Since the poplar will not be particularly showy, this will also work to your advantage.
Good luck and you should be able to work around this with no major long term problems.