Again, it depends on what you are used to or have on hand. I don't have access to a belt surfacer or planer and a lot of large, flat areas in the custom auto trade are faired with a long board to a meticulous degree of accuracy.
The first long board in the photo is spined and dead flat. I chose it because of that and I have had it about 25 years by now. The second one pictured, is also dead flat made from a well seasoned section of 7 ply, BS1088 rated, 9mm marine plywood. The only other purpose to that particular one is it is built to also sand slightly curved areas which is why it is made spineless, but that control has to be imparted in the tool to match the surface. In this case, it's a touch up tool and I have enough hours on the end of that thing to make it do pretty much whatever I want it to.
Epoxy is hard to beat for adhesion both molecularly, and mechanically. In this case it's a mechanical bond on it's own keyed self. At any rate, you would destroy the substrate trying to remove it. Here is a cut away showing how far thinned epoxy soaks into MDF. Approximately 3/16" in the case of these MDF rings. Paint lasts 3-4 times longer over epoxy than just about any other primer. I have proof of that if you would like an example, along with many others.
Most traditional fillers/primers shrink as the solvent evacuates over time. I have seen countless examples of joinery telegraphing through painted surfaces, including two part LPU's. A correctly faired surface should only need a semi-transparent coat of primer/surfacer remaining, yet people lay it on heavy to correct a multitude of sins. It's only really meant for the tiniest imperfections and a minimal tie coat. Epoxy, in this application, tends to take care of it all at once and any shrinkage would be minimal, comparatively.
Again, this is not a suggestion or tutorial. It's just something a bit different than the norm. This particular method just happens to be easy for me, personally. I've had no issues using it.
I have maybe 45 mins in spotting/fairing these cabinets today. They are dead-nuts-flat, including the bottoms, which is even more redundant, considering.
I just thought some might be interested in seeing a different way.