Just to clarify, I dont disagree with Peter J in any way, and I don't think our views are necessarily in opposition as I do agree with his statements. Perhaps I failed in part by not giving any clarifications or limitations to those methods.
I would never try to scrape a curved surface, it only works for flat, but even with roundovers, most of the cabinet is still flat, which is where I use that technique. Sanding is still needed, but scraping leaves much less sanding to be done, at least for me. If your sandpaper is loading up than usually the paint is not sufficiently cured, but scraping can still be performed such as removing a small run or drip, or any tiny surface specks, allowing you to continue with additional coats, sooner than if you waited for enough cure to sand. I find scraping to be a valuable technique, time saving and effective, that many are unaware of.
I lack the chemical understandings to know the failure point of sealing mdf with water/glue. I absolutely agree that products dedicated to that task would be better. Swelling fibers due to moisture absorption is absolutely a concern and in some ways is part of the process with any finish, including on solid wood products. A paint or finish does swell the grain which is why it needs to get knocked back down and made level again, repeatedly.
Understanding those limitations, I've used glue and water under veneer and under paint, and never experienced a failure, but I have also still physically prepped that surface before adding paint or veneer. That recommendation was not intended as a best procedure recommendation but was intended only as a "good enough" in function, which is a category I know many people are in. I have the benefit of two long and stacked shelves full of sealers and finishes but I also recognize that some people don't want to spend $18 for a can of sealer that won't get finished or have any other use beyond that one project. That is the use case for which I was stating that glue and water can be used, but if course I didn't quantify it, which is probably why bad info often gets shared, is outside of its intended context. Material still has to get surface prepped which is the step that can't get skipped of sanding and repeating. If someone has experienced a failure in finish because of water and glue on bare mdf surface id be interested in those details, I just haven't experienced it, but I do put quite a bit of labor and technique into the final finishing process, which there really is no substitute for. Glue and water is certainly not my go to everyday option but I have used it multiple times successfully.
Thank you Peter J for sharing and offering the cautionary perspective. I suppose there is some wisdom in only recommending best practices, but I do find there are also use cases for lesser methods if they can achieve a desired result, when the best case practice becomes not an option.
Lots of good info in recent posts. Mlundy's method is identical to my preferred method for dealing with seams and endgrain, but I could not have explained it as concisely. Seams will show through a finish if not dealt with proactively. If you are going for a smooth paint finish on a seam or end grain, in my experience you must apply some type of filler to that surface and traditional wood filler alone is not enough. It doesn't matter how smooth you sand it, joints seem to always telegraph their way through paint. The wood expands but the the glue joint does not. The area should have a hard resin type of filler applied more broadly and epoxy works excellent.
BTW, I'm old enough to know I've been wrong many times. Ask my kids they will tell you. I see the failure of proposing alternatives as main stream solutions. My intent was to propose a couple less traveled paths of alternatives, but not as the principle methods, which is of course I didn't qualify in any way. Seriously though, when you get good at scraping surfaces it is pretty helpful. I have a lot of power tools but there are a lot of things I still enjoy doing with scrapers, handplanes, chisels, and handsaws, and find them to be very time efficient.