Well, wiping it down really wasn't getting it, so a couple of weeks ago I took the padded accordion-fold mattress out to the back yard and draped it over the stepstool and hosed it real good. I do not know what substance the firm boards inside the mattress are, whether they are plastic or fiberboard or wood or what, so I worried about hosing it, but it seems OK. No warping, and it dried. But either I didn't get it good enough (without soap) or it needs to be done again. I've been waiting for warm weather to hose it again, to be sure it dries. I saw a video where a gal immersed hers in the bathtub, so I'm going to take a chance and try that, because there is still some smell.
If I could get it really fresh smelling, it's tempting to encase it in a clear $6 shower curtain and seal it with lots of packing tape so it will be "wipe-clean" in the future. I can wipe the rest of the playpen sides and bottom and get good results because they are single layer, but the mattress is some kind of firm boards covered by some kind of foam, then covered by the a synthetic printed fabric which is water resistant but not waterproof. Even if it were waterproof, there is potentially access for urine along the edge where he pees, because it is stitched with a binding.
Anyway, depending on how the day goes, today may be the day, or tomorrow. Supposed to be high 80's.
His playpen is almost 5 years old. He was officially adopted on 8/12/21, but he had a month-long foster-to-adopt first, so I got him in July 2021 (Edit to add: 7-19-21), and I already had his playpen in preparation for his arrival. I've been looking at playpens and the biggest issue is most of them have dark colored or splashy multicolored mattresses. You want something light-colored for an incontinent pet, so you can tell whether it's clean(!) That's true of blankets, too, you want a light color, not some dark color that camouflages dirt. Hiding dirt might be OK for a toddler wearing a diaper and shoes, if all you're hiding is footprints, but not for a disabled dog that pees every night. Hopefully Pip can get a new playpen soon, as he has also chewed some small holes in the netting, not too bad...