Just my $0.02,
I have always found that it's easier to make the pattern bigger and remove material to create draft rather than making it to the final size and then trying to add draft and machining stock with Bondo, fillers, etc. as for fillets I almost always use Bondo and a simple form tool, but I have used Drywall compound and found it worked fine but i am rarely patient enough to wait for it to dry.
The trick to using bondo for fillets is not doing it all in one go, mix small amounts and go all around a particular feature, making sure you scrape off all of the excess when it gets to "Green" hardness, then sand it as soon as you can, and absolutely before doing any fillets that intersect with the first one, by scraping off the excess bondo when it is green you reduce your sanding from hours to just a few minutes
My Foundryman always specified Grey automotive primer as the top coat on patterns, after sanding with 450 or 600 wet n dry paper it is silky smooth but not "sticky" as gloss finishes tend to be and it works well over pretty much anything but polystyrene foam, including all woods, MDF, Plaster, drywall compound, metal and of course Bondo(it was made to spray over bondo).
For one offs I always used MDF for my patterns, it's cheap, doesn't give grain issues sands easily and consistently, comes in sheets and mouldings and using the right bearings and router bits you can make a complex pattern by making a single 6mm thick template and laminating it to 18mm sheet after 18mm sheet and simply trimming the thick sheet with the guide bearing running on the template then so on, on each successive layer. The only problem is I have never bought the right router bit instead I have had to make them to suit so a rebating cutter would be reground to have a 20mm height and a suitable taper matching the draft angle, once made such a cutter will do many patterns. This method gives you the ideal shape based on the cross section of the template which will sit at the bottom of the mould, there are complications on internal radii sometimes but overall it works.
I seal it by spraying a coat of acrylic clear over it and then to satisfy my Foundryman the grey primer.
Incidentally, When I say "one offs" we did 400 Propane filler mounting blocks out of one MDF pattern and 200 Boxes for electronic control units on diesel truck engines, the patterns for the filler mounting blocks only just made it with repairs, the ECU box pattern lasted until the foundry burned down in a "Bush fire" we lost a lot of patterns that day
But I do still have a few examples of the Propane filler blocks, actually some of the last ones made, as the patterns were a bit rough at this stage, these two were never used for customers but I kept them for my own use, I will take some photos and attach asap.
I hope this is helpful, The only reason I haven't done any pattern stuff recently is that I just don't have access to my Foundryman mate here, so will need to set up my own foundry and just haven't had time, but after years of all kinds of manufacturing, casting has got to be the most rewarding, So it's just a matter of time I suppose.
Regards
Rick
PS, If you use MDF and have an area that you fear needs to be much harder than MDF so it can last, before sealing the MDF pour cyanoacrylate adhesive(super glue) on and let it soak in this makes mdf super tough
Whatever it is, do it today, Tomorrow may not be an option and regret outlasts fatigue.
