Reply to thread

Thanks for the replies everyone.

So, is it be okay to use the 1/8 " engineered hardboard (masonite) to replace the particle board that I removed? (It is not advertised as acceptable underlayment.) I also thought of maybe using using 1/4 sureply on the edges. I have a 30 inch wide area to fill, perhaps 10 inches wide of 1/4 sureply, then 10 inches of hardboard down the risen center, then 10 inches of 1/4 sureply again to total the 30 inches of width.  This would reduce the amount of trowelable filler needed. Or is is better to forget all of the underlayment and fill it all with a trowelable filler? I have also heard of people using layers of roofing felt to fill areas. What are everyone's thoughts on that? Can it be filled over with a topcoat or is it too soft to provide a base layer?

Thanks again for the guidance.