Timbermate isn't a sealer, but Dingotone is a fairly unique product. As it's also a stain, I wouldn't use a specific sealer underneath it.

I don't know whether Cottonwood (a generic name for several varieties of the Poplar family) is open or closed pore. Does the surface appear to have lots of small holes in it, or does it look smooth? If smooth, you won't need to use any grain filler. Just then make sure you've sanded the surface flat (and don't use too fine a grit, P240 at most, or the Dingotone take-up may be patchy) and removed any machining marks on the edges, and apply the Dingotone direct.

If the surface isn't smooth, then use some Timbermate first.