serious problem with staining poplar paneling

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My husband has began staining his newly installed architectural paneling (did his own work). The backing is furniture grade maple plywood. The rest is poplar. The backboard plywood looks like CRAP!! after he has stained 1/3 of the room. It is more than splotchy. I can't really describe it. Just that it is not a normal look. We are using Wood Sheen, plantation walnut. He has sanded, restained. Looks worse. HELP US!!
Thanks,
Gretchen