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CN, I think the terminology now is getting away from primer-surfacer as a dual purpose product and is more directed to dedicated primers and separate surfacers, at least for the better work. I see that the 1K MP181 is called a "primer-surfacer" and the 182 is simply referred to as a "urethane surfacer". As you say, both of these products require an etch-prime pre-treatment if the manufacturer recommendations are followed. The MP170 epoxy has no such special pre-treatment recommended, presumably due to the superior adhesion and corrosion resistance of the impermeable epoxy. From the standpoint of requiring a separate pre-treatment, I think the term primer-surfacer overstates the "primer" function of the product. I haven't used the 181 much at all having since moved to the 182 exclusively. I have used the similar DuPont 131S 1K "primer-surfacer" quite a lot in years past. Before the advent of dedicated "etch-primers", the pre-treatment was always a two-step phosphoric acid treatment -- a messy, time consuming process to use, for the most part. I consider the 182 (and really any 2K surfacer) to be superior to the older 1K products. There are a number of problems in using the lacquer primer surfacers. One is that they are getting harder to use because of VOC restrictions on the use of lacquer thinner. You'll notice that the tech sheet for the 181 indicates an MR reducer or MS250 "compliant solvent". When I purchased a gallon of 181 once, the local law required I also buy a gal of the MS250 at the same time, which I think is nothing more than acetone, an exempt solvent. The use of the lacquer type surfacers or primer-surfacers also can be troubling. As a lacquer-based product they dry very fast. If the film is applied too dry, it can actually bridge over sandscratches. Unlike a 2K surfacer, the film can absorb solvents from a topcoat and swell and later shrink when it evaporates. When that happens, the bridged surfacer settles into the scratches and later shrinks as the solvent evaporates. This causes the sandscratches to show under the topcoat and spoils the gloss. Conversely, if the film is applied too wet (in the common practice of trying to "fill" a lot), the surface can skim over and appear to be dry but actually the solvent is continuing to evaporate and the film shrink. If it is sanded too soon and a topcoat applied before this evaporation and shrinkage is complete, the continued shrinkage will later also cause the sandscratches to show. This doesn't happen all the time but can be a source of problems if not enough attention is paid during application. The 182 uses no additional solvent for reducing and cures by chemical reaction of the ingredients. After the recommended dry time, the film is no longer affected by the solvents in the topcoat and is swell/shrink-free. At least relatively so. Check out the Martin Senour troubleshooting guide for an official description of the bridging effect, pictorial diagrams and a photo. Look under the "sandscratches" section. The file takes awhile to download but is well worth the wait. I've saved the whole thing to my harddisk for easy future reference. It's the best paint troubleshooting guide I have seen. Beats similar guides from PPG and DuPont. A 2K surfacer will always be preferable to a lacquer type if the proper fresh air breathing equipment is available to safely use it. I don't think the 181 will stick any better, because of "primer" in it's name, to bare, untreated metal than the 182. If you have any sandscratches that need filling after the epoxy, I'd also forget the 181. It can be done but it is simply more risky than with the 182. And if you don't have sandscratches showing, you probably don't need a surfacer anyway. You'll have to make the judgement call of whether not having to bother with your fresh air system is worth a higher risk of problems using 181 instead of 182. I'd stick with the epoxy/182 combo.
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Rod