The early TR4 seats that I used to replace the misfit TR4A seats were very hard to find. When a friend found me a set... they were useable but the seat pans were badly rusted in the cushion pan area and over the years had become cracked in the typical stress points and the seat cushion springs were very weak from heavy rust. I had the seat pans bead blasted by a local guy who also did the welding job to close up the stress cracks around the four mounting holes. I then painted the bright and shinny seat pans with 2 coats of black POR-15 to seal them for life against rust. The seat cushion spring sets were replaced with new springs from TRF... from John Skinner.
The bottom back of the seat pan is lined with a u-shaped pieces of plywood that are used as the anchor points for tacking and nailing on the seat covers. The wood on my seat pans was just crumbling off. So I used some aircraft grade 3/8" plywood to re-line the seat pans with new wood. I cut plywood strips to match what was removed. I then soaked the plywood strips in water so that they could be bent without breaking and would conform to the seat pan shape. They were clamped in place overnight to dry. I then screwed them back in place to match the originals using blind nuts and screws to replace the original rivets.
Recovering Seat Backs
The seat covers from TRF were absolutely beautiful and fit quite well once I got the knack of the job. You apply the new horse hair filler padding to the seat pan back and then apply the supplied cotton to smooth out the horse hair padding. The leather or vinyl seat covers are designed to slip over this stuff and then be stretched and tacked into the wood strip at the bottom.
The seat cushions are pretty much the same deal except the pre-sewn cover is stretched over the spring set with the supplied the horsehair pad and cotton. I ended up taking mine apart again to add a layer of 1/4" upholstery foam on top of the horsehair and cotton because the new springs were attacking by buns. The resulting cushion was very comfortable and looked perfect.
This whole process is definitely doable by most guys. It is mostly a "wrestling a big gorilla" muscle job. And, when you check out the $50.00 labor rate for the local auto or boat trim guy... you will give it a try.