The first issue I faced was how to go about flattening a 37" x 96" slab that had cupped and twisted about 3/8". This is an awful lot of material to remove and a very large area to get flat obviously.
The technique I settled on worked fairly well. Using long winding sticks, I got the ends of the slab in the same plane using both hand planes and my Makita power planer. Then I used a 8' long straightedge and the same tools to connect the ends along the sides. This left me with a rectangle around the edges of the slab that was in the same plane. Finally I just used a straightedge side to side along the length. It worked.
That left me with a slab that was quite flat but with a lot of cracks and knots and substantial tearout. This is the point at which you want to use epoxy to fill in the cracks and knots. I chose to use T-88 epoxy, which I had on hand, because it disappears under varnish and dries very slowly. The problem I encountered is that it dried so slowly that it got absorbed and the level would drop below the surface. In places it ran completely through the slab onto the floor. To remedy this, I taped the cracks and knots on the bottom and overfilled the cracks and knots but the epoxy would still soak in so much that I had trouble maintaining with absorption. Finally I mixed fine sawdust into the epoxy and this solved the problem. I am not sure how else to do it. I think a faster setting epoxy might be better. It was a lot of work taking the overfilled epoxy down to the level of the slab. The epoxy fill actually turned out much better than I expected. Especially with the sawdust, it blends in quite well and looks good.
As I wrote earlier, I couldn't figure out a good way to deal with all the tearout. The hand tool that worked best was a cabinet scraper but it took forever because of the depth of the tearout. I finally gave up and turned to a belt sander, which I haven't used in years. I got better at it eventually and, by keeping it moving, I was able to smooth the slab without introducing too must unevenness. I started with the bottom, so I am hopeful that I can do better on the top. What I may do is use the belt sander to get as close as possible and then spend a lot of time with the cabinet scraper. If you know of a better way, I 'd like to hear it. As I have told you, a plane, no matter how sharp, will simply not work because of the soft douglas-fir and the swirling grain.
The epoxy fill actually turned out much better than I expected. Especially with the sawdust, it blends in quite well and looks good.