Actually I wish I could buy HDF (high density fiberboard), because I actually reach the limits of MDF's internal structure when I "sculpt" it like that ... The surface gets almost fluffy and fragile, but the resin I put on it makes it a lot more tough (and that resin can be sculpted too ... in a way) Anyway but the shop where I usually buy everything can't have any HDF ...
I know exactly what you mean! That happens to me all the time, specifically when I'm sanding sword blade tips. They harden up once I get some gesso on em, but I can only imagine how much fluff you're getting with all of that carving. Good luck to ya man!
Also, I'm sure you could order some HDF, ask around for some trusted sites. I don't use them because all I've needed so far I can get at the local hardware stores, so I can't really help you.
I had also that problem when sanding sword blade tips : and even with gesso it remained fragile. But I found the solution not long ago with the "liquid resin" I apply everywhere before applying gesso. It's really usefull especially on sword blade tips and edges : makes them much more tough than with just gesso (but I still use gesso afterwards, as primer for acrylics paint)
Gesso is a wonderful thing! How heavy do most of your props turn out? Because a lot of my MDF pieces end up being on the heavy side. That might also be because they're ridiculous weapons, but I was just wondering if you've noticed that as well. Also, does your resin end up weighing it down noticeably?
No my props are't really heavy. But for the ones that have huge dimensions (like my Cloud's sword), those ones get hard to hold straight when you hold them from the handle. I don't have that problem with a bow because you hold in the centre, and most of my props aren't that big anymway.