A few comments looking at the rebated board below. The edge cut is indeed clean (good work!) and that's what matters. Patching torn up edges is much more difficult than flat surfaces. Those shallow accidental cuts are best filled with a polyester resin filler commonly/generically called Bondo in the US. I use the nice flat edge of an old Stanley blade to fill light voids like that. Do an initial application that's smooth and
level with the surrounding surface. Yeah I know, the tendency for advice will be to slightly overfill it just above the surface and sand it back to smooth. The problem with overfilling is that the effort will likely yield bumpy messy ---steaming pile of meadow muffin--- with a whole lot of sanding to follow! Just get a nice smooth surface that when set, is slightly shy of the top surface. Sand this back to smooth and then smear a second fill layer over the top, again with a blade or similar. Sand that back with 400 grit and voila! void gone, nice smooth surface to prime.
So long as the filler primer you are using is
not water based several coats and sanding back with 200 then 400 grit will get you where you need to be. Water based anything and MDF are not a good match. Water based finishes take too long to dry and inevitably swell MDF.
Some extra thoughts: When I talk about acrylic paint I am not talking about latex/water based paint. I only ever mean solvent based automotive acrylic lacquer. This is thinned with acetone or mixed acrylic solvent.
The quality of polyester resin based filler or car body bog/filler can vary a lot. Cheap bog is terrible ---smurfy--- stuff. Lumpier to work with and harder to smooth. Premium quality or heavy duty filler like Selleys Plasti-Bond is what I use. Costs more, but well worth it. Easier to get really smooth, less air pockets, sets harder and tends to expand and shrink with MDF better, especially in varying or changing temperature conditions.
Hindsight is all well and good, ha hah, a sandwiched composite board approach would have worked too! Doesn't matter, to my eye the work you've done is good. Overall this design is a really cool idea. A worn metal look will really suit it.
