IMO, there is no free lunch.
If you want smooth and lump free, it's going to take a lot of attention to detail and prep work, no matter what substrate you choose (wood, PMF, glass weave, etc.).
Even aluminum (and aluminum faced composites), Filon and the back side of FRP (which isn't really that smooth) will require effort to achieve a good fit and finish; and these all require some sort of trim or additional hand work at the edge joints. Since I don't like the thought of potential leak points in trim, and (on paper at least) trim adds cost, I didn't go that way.
Even if a textured surface is desired, like with canvas or glass weave, it takes a lot of attention and care to have a good base, and uniformity at seams/overlaps and edge transitions/wraps. Then, if you end up using a little putty (filler or whatever) you end up with a smooth spot that breaks up the continuity of the texture (catch 22).
Like any painted (or other high finish) surface, the final result is only as good as the prep work and attention to detail that goes into all of the stages.
Face it, body work sucks. There, I said it.
I'm doing epoxy and glass over foam and wood, in a theoretically monolithic outer skin shell, with rounded over profile edges to eliminate trim. After getting the base reasonably fair and smooth, and laying up two plies of glass with the profile corners wrapped, I'm at the stage of filling the weave. I'm using West System's 410 filler troweled on. Not the best solution for vertical surfaces; if you don't get the amount of thickener/filler right it tends to slump. I haven't tried spraying high build filler/primer, but keep threatening to in my mind (... I had already bought a large amount of epoxy, so am continuing that way instead of spending even more money on spray gun and supplies). Mind you, I'm not going for a killer automotive grade paint job, just smooth and even with no glaring garbage.
From the perspective of a novice paint and body worker, I can fairly say that if you want it to look good, regardless of which media you choose, plan on more work than you might think initially.
Sign me,
KC the wanna be pseudo perfectionist